Why does the point of impact shift from one side to the other?

by Tom Gaylord, a.k.a. B.B. Pelletier

This report is for blog reader Roger, who has this problem, and also for RifledDNA, who says he has it, too. Fred DPRoNJ (Democratik Peoples Republik of New Jersey), also expressed interest in the topic. I suspect that hundreds of our readers, if not thousands, are curious. Why would a scope that shoots to the left of the aim point at 10 meters be dead-on at 20 meters and off to the right at 35 meters?

Here’s part of what Roger told us:
“This time I’ve scoped a S-410 and something strange is happening.
 For example: I zeroed the scope for 11m, thus the far zero would be around 44m. When shooting that distance although the elevation is fine the POI shift to right around 7cm.
 If I re-zero the scope in 44m and shot back to 11m, although the elevation is fine the POI shift to left around 4cm.”

Roger doesn’t state the problem in quite the same way that I do, but it’s the same problem. Gun’s POI changes as the distance changes.

The answer is in the alignment of the scope with the gun’s bore. But before I get to that, I want to address an answer that someone else on the blog gave to Roger. Someone guessed that his problem was caused by spiraling pellets. Pellets sometimes fly on a spiral path as they travel downrange. I wrote about it in this report: Do pellets spiral?

That answer was correct, as far as it went, but there’s one way to diagnose the difference between spiraling pellets and an alignment problem. Spiraling pellets move back and forth from one side of the aim point to the other and back again as they go downrange. Pellets shot from a gun with an alignment problem do not. They’ll start out on one side of the point of aim and move to the other side as they go farther from the gun, but they’ll never come back. It’s a one-way trip for pellets shot from a gun that has an alignment problem.

Don’t blame scope shift
Too many airgunners are willing to blame problems like this on their scopes. They call it scope shift. Real scope shift is rare, though not unknown. If you want to learn more about it, read my article about scope shift.

Scope and barrel must be aligned
The scope and barrel must be pointed in the same direction for the scope to work perfectly. Because the scope has some latitude of internal adjustment, it’s possible for the scope to be out of alignment with the barrel and still have them coinciding at one distance. That distance is called the sight-in distance.

But if they’re not aligned, the shots will be off to one side when they’re closer than the sight-in distance and to the other side when they’re farther away. The drawing below illustrates what this looks like, although I’ve exaggerated the offset so you can see it. In reality, both the scope and barrel may appear to be perfectly aligned.

scope alignment
The axis of the scope (at the bottom on the right) and barrel are offset in this drawing, but the scope can be adjusted so they coincide at one distance. Any closer or farther away than the sight-in distance, though, and the pellet will move to one side of the other. This drawing is grossly exaggerated to highlight the problem in a short space.

How to correct it
To correct this problem, the scope must be positioned on the rifle so its axis is parallel to the axis of the bore. This may be easier than it sounds. If you have 2-piece scope mounts, for example, you can turn each of them around backward (one at a time, of course) to see if that corrects the situation. It often does. You can also swap the front ring for the rear ring for further improvement. There are 4 different possible combinations of scope ring positions with 2-piece mounts (scope rings). If you have 1-piece mounts, all you can do is turn them around backward. That’s why I prefer 2-piece mounts over 1-piece mounts in most situations.

If changing rings isn’t enough to correct the problem, you can shim them. Shimming the rings for sideways correction is the same as shimming for elevation correction — you’re just moving the scope in a different direction. A word of caution — when you shim the rings for sideways adjustment, be careful not to extend the shim too low on the ring; because if it gets under the scope, you’ll cause an elevation problem that didn’t exist before.

What about optically centering the scope?
Optical centering will not solve an alignment problem unless the scope really is in alignment with the bore in the first place, and it’s the internal adjustments that are throwing it off. It’s worth a look, but the odds are it will not be the solution. The best approach is to start out by optically centering the scope to eliminate this from being a cause of the problem.

Now the bad news
Sorry, but almost no scope is ever completely aligned correctly. It’s nearly impossible to do so. The target scopes use bases (and rings) that are separated by much greater length than the scopes we commonly use to minimize the effects of misalignment.

If a scope was perfectly aligned, it would be centered at 10 feet and again at 200 yards. I’ve never seen one that was. They always shift a bit as the distance changes. But they can be very close to the centerline and nobody will be the wiser. You just have to re-zero when you change from a 100-yard zero to a 200-yard zero. Or, if you shoot an airgun, when you change from a 20-yard zero to a 45-yard zero.

But the problem that started this report was a 7 centimeter (3 inches) shift to the right at 44 meters when the rifle is zeroed at 11 meters. That’s too much of a shift and can be corrected by the methods described in this report.

The good news
The good news is that a majority of airgunners will notice this phenomenon and may eventually learn how to deal with it. But the majority of firearms owners don’t even know it exists. They’re still in the world of “bad” scopes that they sell to buy name brand scopes that have to be trusted because they cost so much! In the end, you’re better off knowing the truth.

99 thoughts on “Why does the point of impact shift from one side to the other?

  1. I had this problem with my SLR 98 last year. You instructed me, I fixed it, my life got a little better. Thank you so much!

    Also, are you interested in the new FWB 124? I was very glad that Walter had such a success with the LGV, I look forward to the new FWB’s sporter. I’m glad spring guns will be around for a long time.

    • Pop’s Slr,

      If you’re referring to the NEW Walther LGV…what information do you have that indicates this new fine springer has been a success?

      I’m starting to wonder if high end springers are dead.


      • Nothing solid…but Walter is making an underlever version, and a 17 ft lb .22 version, and FWB is getting back into the spring game…the market seems to be sustaining development and innovation. Conjecture on my part.

  2. You know that also happens with the up and down POI (point of impact) also. Its usually over looked because people think it is just related to the range difference.

    But if you have bad scope alignment up and down you will also see a greater difference in POI.

    That makes a big difference when hunting and shooting at different distances.

    If I have the scope right in that axis also I dont have to hold over or under as much. And it allows me to stay in the kill zone better. Not as much deviation.

    • If your unable to correct the alignment enough that it’s unnoticeable would you be able to adjust your windage so that it at least runs parallel and consistent, just account for it on the reticle?

      • RDNA
        I think your talking about like if you have a reticle that is broke up in mil dots or even 1/2 mil dots. That’s the kind of scopes I use.

        What they call windage hold is your left and right and it has been nick-named Kentucky windage. I guess it got that name because when the wind would blow from right to left or vise versa; the shooter would hold the sight over towards the direction the wind was coming from. Of course you would have to hold more or less depending on how hard the wind was blowing. Same for if you shoot at different distances. And some people use the scope clicks to do that. But in a hunting or pest control situation I don’t have time to click a scope adjustment. I use the hold over or under depending on range. And same for the wind if I see it moving some grass or tree limbs by what I’m shooting at. And truthfully I want my scope to stay zeroed at a certain range on all my guns. When you do that you can start getting to know if a guns not shooting the way it is supposed to. Pellet weight and the feet per second the gun shoots at will make the hold over or under be different from gun to gun.

        That’s why its important to get the scope axis set. The better you set your scope up the easier it is to shoot at all distances. Then once you shoot the gun enough you start to know how high you need to hold for a certain distance. Its easy to do with 1/2 mil dots. Shoot at a distance farther out than your zero range. More than likely the POI of the pellet will be low. So all I do is put the reticle on the POA (point of aim) then see how many half or whole mil dots the pellet hit low. Then I know at that distance that’s how much I have to hold over. So I aim at that same target holding that many mil dots up and you should impact your target that you are aiming at that greater distance.

        Then on another note that’s why its important that you can shoulder the gun and rest your cheek on the stock and not have to move around to get your sight picture in the scope. If you can’t repeat your hold and look through your sight the same way you will never shoot a good group from your gun. or you will never place that shot correctly when you hunt or pest control. Repeatability in your hold is what will help you be precise.

    • Vertical mis-alignment is not a problem. Many, if not most, truly long range rifles are scoped with 10, 20, or even 30 MOA bases. Their scopes are deliberately misaligned with the barrel so they don’t run out of scope elevation adjustment range when reaching way out there!

      The problem in windage is not just the angular misalignment of the scope tube with the barrel. Rather, it’s the scope’s horizontal offset from the bore center line. Put a scope mount on an M1 Garand and you’ll have the same problem, even if the scope tube and barrel center lines are perfectly parallel. The scope sits off-center and to the left of the en-bloc clip port on an M1 Garand. When mounting a scope horizontally off-center like this, you create the same problem in windage that we have learned to compensate-for in elevation. We’ve learned how to compensate in elevation, due to the necessity of mounting scopes vertically offset from the bore center line. (A scope tube and a barrel cannot occupy the same space!) Personally, I like to set up a Garand scope to shoot a POI that maintains the horzontal offset at all ranges. Then, if I want to “hold some windage,” to compensate for it, it’s the same inch or so at all ranges and it doesn’t grow at ranges well inside or outside of my zero.

      The reason the scope center line vertical offset (offset in elevation) is not a problem is we expect it and we’ve learned to deal with it. A projectile trajectory is expected to deviate vertically from the line of sight. External ballistics calculations and tools like a dope card or mil-dot master (or just simple hold overs) permit us to shoot accurately over a wide range of target distances. Though not a concern at long range, the trajectory also moves vertically from one side of the line of sight to the other and back again, as it crosses the line of sight twice, (It crosses twice instead of once, as in BB’s example, because gravity acts on the projectile.) Every air or rimfire gunner knows this well. It’s a deviation that is expected vertically!

      The same behavior is NOT expected in windage and, generally, only wind (and sometime Coriolis) is accounted-for. We expect a gun to “shoot straight” at different ranges in zero wind. This is the problem with the misalignment/offset error.

      I really like the Burris rings with the Pos-Align inserts to easily achieve horizontal alignment and centering. I’ve never tried them on a springer, however. I suspect they would work and they don’t mar up your scope tube either.


  3. Also some guys were talking about tranny fluid and such for sealing co2 guns, have any’a’yous evah (Rickaah where you at) tried permatex ultra disc brake caliper lube? Says its good for pistons and rubber o-rings and its kinda thick and….. I like it. Any reasons I shouldn’t use it?

      • I think I would stay away from using it anywhere on a sproinger or PCP where it would come under high compression, such as the breach seal, piston seal, etc. I had PA install a gas spring in my Gamo CFX a while back. When they reassembled it, they apparently got some grease or such in the compression chamber. I had a massive detonation which sounded like a .22lr going off, a good bit of smoke and no more piston seals. It also sent my breach handle to who knows where. They were nice enough to rebuild my CFX for me. High pressure air and petroleum products do not mix well.

        • The lubricant RDNA is referring to is silicone based, so detonation should not be an issue. I did have a coworker advise not to use it as a lubricant because it contained silica, ie ‘sand’, I used it anyway with no ill effect and have also rebuilt many hydraulic assemblies with it. As far as the transmission fluid goes, I trust it. I bought some Bars tranny sealer to try on a 3 ton floor jack. Hopefully the $4 investment will save me enough time & money to fund a day of shooting. If I have any problems I will advise against it, but I don’t see that happening.


          • I don’t know about the silica though probably the crystalline molecules would be thin and small enough to stick and slide anyway. It had a very unimpressive increase of combustion, hardly a detonation. I have not used it But once and its so thick I imagined a long term lubrication being accomplished with it. So far so good. And Calin, that holding of consistent horizontal offset was what I meant by compensateing to at least keep it parallel. Are you saying you can run the scope axis parallel to bore axis? If not ever intersecting they can be made to run evenly with each other right? I don’t know, Bb said no… maybe he didn’t understand what I meant because it sounds easy enough, I probably said it stupid.

            • I agree with B.B.–adjusting the reticle on a scope doesn’t align it and, in the case he illustrated, it can’t fix the problem of not being able to obtain a consistent windage zero. (The zero can only be obtained at a single range.)

              However, if you are willing to abandon your entire desire for a windage zero (and your desire to have the point of aim (POA) and point of impact (POI) coincide, the reticle can be adjusted with the windage knobs to make the line of sight (LOS) parallel to the barrel and obtain the situation you described (and I described in my M1 Garand scope example, which I included mostly as a thought experiment for others). In doing so, your windage is not “zeroed” at any range. The LOS remains parallel to the trajectory path at all distances.

              You can do the same for elevation, but most people don’t find it to be useful, because we have to deal with bullet drop, regardless of how we adjust our scopes (unless you are shooting straight up or straight down ;)) and we like being able to hold dead-on at the zero ranges while trying to get the POA and POI to coincide. If you align the LOS with the bore in elevation, you’ll need holdover at all ranges–the least holdover occurring near the muzzle, where only the LOS offest (scope height) is a factor and the bullet drop due to gravity is zero. You’ll never need to hold-under, because the trajectory path will always remain below the LOS. Some ballistics software can compute this case as well as the conventional zero case, and provide the trajectory path deviation from LOS, with holdovers and deviations in inches, MOAs, mils, etc.

          • Reb

            I am all for DIY and self sufficiency and saving money. And I am all for you having extra cash for a day of airgunning. But the thought of you using transmission sealer in a 3 ton floor jack (I am supposing to cure a leak) made me wince. Then I cringed. If I think about it some more, I just might get the vapors.

            Perhaps you were yanking our chains and perhaps I don’t know what the heck I am talking about. If not, may I humbly suggest you repair the jack the conventional way, with new seals? Perhaps a new jack is in order? Safety just isn’t the place to save a few bucks.

            It is difficult to enjoy a day of shooting while pinned underneath a truck.

            Happy/Safe shooting brother.

            • No chain yankin’ here,..I’m gonna do it & you can’t stop me!
              in the late ’80′s I got some fork oil with O ring expander to fix the leak @ the Yamaha shop (recommended by the dealer). They never leaked again, over 20 years. I’m glad I trusted that old biker, he saved me a lot of time and money. I also learned a lot more from that biker because I listened to what he had to say. That being said, rest assured I will not be jeapordizing any body parts in this experiment.

              Here, -Hold my Beer!


          • Sand is mostly silicon-dioxide, aka quartz.

            Silicone is polymer of organic compounds on a silicon-oxide backbone.

            Silicone contains “sand” only if you believe that petroleum oil (various chains of carbon with hydrogen leaves) contains graphite and diamonds (both forms of carbon) or COAL (a less pure form of carbon)

    • RifledDNA, sorry been busy, so i have heard a drop or two trick using transmission fluid in co2 applications as it does cause swelling in rubber in general. i have also read BB talking about a couple of different tricks, but i have never tried them. I only own 1 co2 pistol and it holds well for a long time but is starting to loose pressure over a smaller window of time now. Personally i think rebuilds are necessary with the possibility of blow outs or high pressure co2 loss with weak seals (scary), especially if you own nice guns but if you needed to in a pitch because you found a gun with bad seals but wanted to make sure it fires before buying and servicing, then it is worth a try. I would use a better product also, maybe something designed for that specific purpose. Now back to my not so fun chores….

  4. Maybe I’ll start shooting only at 10m to avoid this problem:-)

    Where does the majority of the misalignment originate? Is it the alignment of the bore to the barrel, the barrel to the receiver, the scope grooves to the receiver, the mounts to the scope grooves or the optics to the scope body? I understand the vertical misalignment as coming mostly from barrel droop caused by misalignment of the barrel and receiver.

    • Somebody can correct me if I’m wrong. But its all about keeping the center-line of the scope parallel to the center line of the barrel on all axis.

      And all those things that you mentioned can be off but bottom line is to get the scope lined up to the barrel. Its usually done by correcting the way the scope is mounted.

      • No–only the horizontal alignment matters. The scope and barrel bore can only be aligned horizontally (as in BB’s windage error example). Even if the center lines are perfectly parallel in both planes, the scope center line must be “optically angled” from the barrel center line, to achieve an elevation zero, Physically angling the scope or optically adjusting it to “angle it” amount to the same thing geometrically. Also, it’s the offset that matters. Consider an M1 Garand scope that is perfectly parallel to the barrel! See my post above.

        • CB
          I read your reply above and know what your saying. But I do know it makes a difference also vertically.

          I have mounted a scope and zeroed it then shot the gun at different distances at a target (closer and farther distance). Then came back and shimmed the scope up. Zeroed the gun again at the same distance as I did without shims for elevation (my side to side was ok). And then shot at the targets again at the different distances and the farther the distance I was from my zero point(closer or farther distance). The pellet was farther off with the shimmed scope when I compared it to my first pellet I shot without the shims (verticaly up and down it was off more).

          So yes like you said we have become accustomed to the elevation hold over or under. But there is a difference when you raise the scope. Maybe some body that shoots out at 100 or 200 or even 1000 yards I guess would shim there scope vertically. But when I shoot my air gun out to about 50 yards or closer most of the time with the exception of some 60 to 70 yard shots. I want my scope true. I guess maybe that doesn’t matter to some people on the vertical scope alignment. But to me it does.

          Have you tryed the experiment before that I just explained?

          • No–I’ve never noticed a difference in adjusting elevation with an angled scope mount rail vs. just dialing more on the scope turret knob. (EGW 30 MOA rail/Vortex Razor 5-20×50/Seekins rings/custom wildcat 7mm). It takes the same number of MOA/mils either way. I never noticed a difference checking my reticle elevation setting with my Leupold Zero-Point either. If the reticle gets pointed down 30 MOA, then the barrel will be angled 30 MOA higher when the cross hairs are on the target–it doesn’t matter how that 30 MOA down is achieved.

            The only things I can think of that might explain your experience is your scope height also changed, which greatly effects trajectory at short range with a precision pellet or rimfire rifle, or the scope adjustments aren’t constant across their adjustment range. What you described is consistent with a zero at the first trajectory’s crossing of the LOS (a short range zero) and an increase in scope height accompanying the shimming–at least until past the second trajectory crossing of the LOS (the second zero).

            • CB
              I agree with you about how the clicks may not work out to be true.

              I use the Hawke scopes with the side AO adjustment on all my guns now. Two of my guns have the 30mm tube scopes. And 2 of them are high power scopes and the others only go to 12 power. But that’s all besides the point.

              I still have seen the POI change more at different ranges after raising the scope in the rear with shims on different guns with different rings. So something is happening. And it could be very well that it does make a small change in scope height so that could make the amount the POI changes be affected more at different ranges the farther you are away from your zero point.

              I would say also that the variation I see probably wouldn’t even matter to some body else and I bet they would never even know that existed on the vertical POI unless they did some testing.

              At this point in time it really doesn’t matter to me what is causing the vertical shift that I see. I just know it happens. And I know what I done in the past to correct whatever it that’s going on.

              Oh and I made a pretty simple tool that I mount in the scope rings that are mounted on the gun. Its basically a barrel with 2 collars on it that you mount in the scope rings. The barrel slides forward or back and can be rotated. I put a indicator and magnet on the end of the barrel and touch off to the barrel on the gun. I just slide the indicator forward and back and then do the same on each side of the barrel. I can tell exactly if the scope ring bore is of to the guns barrel. And yes I made sure the barrel I used as the tool is straight I checked it at work. And the collars are about one and a half thousandths bigger than the barrel diameter.

              Oh and I do use a bubble level to set on top of a flat spot on the scope to make sure the reticle is level. All I know is I try to take all the variables out when I mount a scope. I would rather have the scope mounted true. What ever way some body sets up a scope for their uses is up to them. And I have seen all kinds of different things brought up in the hobby’s that I have done. And everybody has a idea and their own way. All I can say is what I have seen.

              • >I would say also that the variation I see probably wouldn’t even matter to some body else and I bet they would never even know that existed on the vertical POI unless they did some testing.

                Yes-my observations were on a long range rifle. From running ballistics programs, I can tell you that the benefit of low scope and line of sight height only comes into play in when shooting close to the first trajectory zero distance or in front of it (common in airguns, rimfires, etc.).

        • CB
          And maybe you mis-understood what I meant. Yes the quickest way to determine that your scope is not true to the barrel is windage. I wont shoot my gun if the POI is different at different distances in the horizontal movement. I could live with vertical being off but I don’t accept that either.

      • B.B.,
        Didn’t you once show us a target rifle you built for Edith in a past blog where the scope was horizontally offset to allow her to hold the rifle right-handed but sight with her left eye? In this case, the scope is mounted parallel to the barrel, but the horizontal misalignment only allows the poi (point of impact) to coincide with the POA (point of aim) at the one distance where the gun is zeroed, as Cal was pointing out.

  5. Good morning everybody, and thank you B.B. Having just fixed up another .22 BSA Meteor i fit my AGS 3-9×40 scope and took it down the woods to test it for accuracy, and sighted it in at 10 yards getting some lovely ragged one hole groups with ten Superdomes. But when i tried at 25 yards the groups were 1 1/2 inch and over to the left by about 2 1/2 inches, so i rezeroed and accepted the smaller ten yard shift in POI. So thanks for todays blog as it looks like i can fix the problem now, it seems you learn something new everyday. The only problem i need to sort out now is that the trigger pull is something akin to that of a Norica Quick, looks like I’m going to have to do some polishing with the Dremel and adjust the trigger as well as shim the sides for the side play.


    Best wishes, Wing Commander Sir Nigel Tetlington-Smythe

  6. Thanks, B.B. Great topic.

    I think another piece of the scope mounting puzzle that can cause this sort of problem is having your scope reticle out of level. With a canted reticle, you’ll be okay at your zero range(s), but holdover and clicking for other ranges will have you impacting right and left of your zero.

    I’d be curious to hear how the veterans here level their scopes when mounting. Better yet, what are folks’ scope mounting rituals in general?

    I myself haven’t yet graduated beyond eyeballing the reticle level, and I’m sure I suffer some from it, at least on the FT course.

    Bubble levels? Scope/bore alignment tools? Plumblines?


    • I’ve used bubble levels for years with fair results, but it sure would be nice to have a graduated scale-bubbles can ‘stick’, and since they’re never as large as the centering lines you still wind up in a guessing game by eyeballing the gap on either side.
      Works for minute-of-deer, but probably not the most effective for precision target work.

    • I’d love to read a B.B. piece on the subject! I suspect the most accurate level reading can be obtained with a good machinist or gunsmith bubble level. However, I use a digital Smart Level for its ability to read off-level. Once I have my gun strapped down and secure in a gun vise, I can match the digital reading from the receiver (usually obtained before the scope is put in place) to the scope. If I want to take the time to perfectly level the receiver (some gun vises provide adjuster feet), I think a very good bubble level can be more accurate. However, the Smart Level can easily detect scope tube twisting as the rings are tightened, which is an unfortunate liability of many rings. Even rings with symmetric screws can be prone to it. I usually find that the scope turret cap is a reasonably flat and well-aligned scope surface on which to rest the level. From trials, I know that either type of level can better what my eyes alone can do–mostly because my eyes can’t identify the gun vertical line terribly accurately, whereas the scope center line is easy to see through the eye piece. ;) I don’t have a smartphone or PDA, but many of them have accelerometers in them and can run smart level type software. I don’t know if they are very precise, but it’s an idea.

  7. I agree with Calin Brabant. I use Burris pos-allignment rings on many of my rifles. In my opinion, they are one of the finest scope rings ever designed. I hope that BB will try them , and give us his opinion. The worst example of scope shift that I ever had was with my Rem. nylon 66 rifle. The sheet metal breech cover had scope mounting grooves milled into it. They were so badly misaligned, that scope shift was 3/4″ going from 50′ to 75′ . I wish that I still had that rifle, so that I could see if the Burris rings would solve the problem. Ed

    • zimbabwae ed,

      This isn’t the first time someone’s mentioned the advantages of Burris Posi-Align rings. I’ve sent an email to Pyramyd Air’s tech and purchasing departments asking them to consider stocking these.


      • B.B.,

        Please note, the burris signature rings with inserts either come ready to mount on dovetails (with 4 options of clamping feet) or come ready to mount on a weaver/pictinney.

        The burris signature rings for dovetails is sku 420554. The burris signature ZEE rings are for weaver/pictinney. Sorry don’t have a sku. They both come with the inserts so the rings won’t scratch your scope but these inserts are “0″, neutral for adjusting windage/elevation. In other words they don’t provide any windage/elevation adjustment.

        You need to buy the inserts that adjust for windage/elevation separately. They come in many sizes. One option is to buy the kit that contains a variety of inserts in a variety of sizes, sku 626016.

        All of the above information is for 1″ rings.


          • B.B.

            There is also another factor to be considered: the parallax adjustment ring!

            I was testing a FX rifle with a 3-9×40 scope at 27,3 yards (25 meters) with according parallax adjustment.

            Then I tried at 54,7 yards (50 meters) with the parallax still set at about 27 yards. The hits in the target were of course low at the target, and I noticed that the target seemed a bit unclear. I had forgotten to adjusted the parallax to 55 yards. I changed it, but the pellets did not hit in the same spot as before. Now they went far out right and up!

            Then I tested at 27 yards again, and I was still far out right and up. I found out that slightly adjustments of the parallax ring actually moved the crosshair all over the target. The new scope branded (read: Chinese made) Walther had a defect parallax adjustment.


            • GH

              I have heard of scopes changing P.O.I. when you make adjustments on the optics or zoom. Some seem to be worse than others.
              I have seen some people suggest to always make adjustments in the same direction. Example…
              adjust farther than you need to , then bring it back to the right point. always do this in the same direction.
              Someone must have found this to work in some cases when the scope does not seem to work right the way everyone thinks it should.


  8. Excellent blog, BB. Never occurred to me that the scope would be off-center from the bore. I only considered elevation to be a problem. Case in point, when re-mounting a scope back onto my Talon, the rifle shot low at 28′ and I did not have enough adjustment in the scope to get it to POA. First fix attempt – reverse mount (one piece BKL) and Bingo – the POI was slightly left of POA. Five clicks on windage and all was right with the world.

    Fred DPRoNJ

  9. OK, dumb question time. Could this also happen with “Dot” sights? What about projection “halo” sights? If so, wouldn’t it be possible, yet rare, to happen with open sights? Seems as though any sight could do that if it’s not true with the bore.

    • Common (rifle) open sights have both the front and rear sights mounted on the barrel, or as close as possible to such. That tends to mean that, unless the barrel is bent mid-stream, the sight-plane is aligned over the bore.

      Any sight in which the a sight component is NOT attached to the barrel can result in a sight triangle, in which the line-of-sight cuts across the two legs formed by the receiver and barrel. Even a receiver peep sight will have this problem (and maybe even worse as the hypotenuse is rear peep to front sight, and not rear to sight-in distance).

    • Looks like it can happen. I grabbed the 953 the other day so I shoot a really nice group @ 10m. I must,ve left it set for 30m because on inspection the group was about 6″ high. More interestingly the group also shifted to the right about 2″. I haven’t messed with it since, I ended the day shooting the 392. I was using a cheap Daisy electronic point sight & had considered an alignment problem. I think I’ll test it tomorrow.
      Thanks for this write up B.B.!


  10. BB, Edith, The Burris rings come without the offset rings. They come in a set, .05, .10, and .20 offset. They have to be ordered along with the rings ( they have 0 offset inserts .) Thanks for evaluating these rings. Ed

    • I have both the 1″ rings and the 30 mm rings. My rings came with a full set of inserts, including the offset ones. For unknown reasons, the 30 mm rings only come with zero and +/- 0.010 inserts but they are still usually enough to align a scope or provide significant elevation adjustment for long range shooting. The amount of adjustment available depends on how closely the rings are mounted, of course. Also, the manufacturer’s web page indicates that extra ring inserts are available individually, or in multiples in sets and “gunsmith” kits (at least for the 1″ rings). -Cal

  11. This would cause consternation. But I’m not surprised that the scope and barrel are not aligned perfectly in the horizontal plane. It sounds like the solution is essentially the same as scope adjustment in the vertical plane: correct within the scope adjustment or shim. Unfortunately, I don’t think this explains my very weird problem with my Anschutz which will be dead on at 50 yards and then drop several inches at 100 yards. Very strange.


    • Most likely your Anschutz is chambered in .22 long rifle. Which is a low(er) power cartridge compared to say a centerfire caliber.
      If you were to shoot a centerfire caliber at 50Y then move on to 100Y, you might see .5″ drop, and that is to be expected because of bullet drop from gravity. But a .22lr’s bullet mas less mass so it has less inertia and is affected by gravity greater, so it may drop several inches between 50Y and 100Y.

      My Anschutz 1907 (in a 1914 stock) can shoot dead on at 50 feet, then when I move on to 50Y It will shoot right on because of it’s secondary zero point. Then farther out the bullet drops more because of gravity, and it 100Y it will shoot between 1-5″ low depending on the cartridge.

      What your experiencing is common bullet drop, every projectile is affected by and should be expected.
      Hope this helps.

  12. Thanks B.B. for another informative blog, sounds like its optics week! I got my UTG DTP Rail adapters, they are awesome little contraptions, I ordered two sets so Id be able to put them all the way along the mount base, they fit like a glove, bite down like those cool jungle ants that you can suture wounds with, don’t raise the mount at all. Getting it sighted in now, it seems to be lined up with my bore a thousand times better too. I would recommend these to everyone that needs an adapter but doesn’t want the scope raised, they really work mint. Time to go shoot!!

  13. B.B.
    Your article said “You can also swap the front ring for the rear ring for further improvement. There are 4 different possible combinations of scope ring positions with 2-piece mounts (scope rings). ”

    Could you please elaborate on this? I am a little lost, thanks.

    • Joe,

      Here are the options.

      1. Turn the front ring around so it faces the other way.
      2. Turn the rear ring around so it faces the other way.
      3. Swap the turned-around rings (front goes to the rear and rear goes to the front).
      4. After the swap, Turn the front ring around so it faces the other way.
      5. After the swap, Turn the rear ring around so it faces the other way.

      Ooops! I guess that’s 5 ways, not 4!



      • B.B.,
        I guess what you are saying is that the rings may have caused the misalignment, and by swapping, it may solved the problem, correct?
        If this is the case, then the two rings were not made exactly the same.

  14. I should rig up a photo shoot of my ancient Marlin Glenfield 60c…

    When you look down on the stock, you can easily see that the barrel angles to the left relative to the receiver. The stock inletting is maybe a quarter inch on the right, but only an eighth inch on the left (at the muzzle end).

    But the receiver is square to the stock.

    I’ve got Dr. Scholes Moleskin pads on the scope mount to angle the scope to the left, but that still doesn’t align it to the bore (that would require the rear scope ring to hang over the right edge of the receiver).

    Result, it can be zeroed for one distance, but anything else is the line of sight being the hypotenuse of a triangle.

  15. So I finally have the Bushnell 1.5×4.5 in a Colt competition one piece mount attached with the UTG DTP adapters on my Blackhawk Elite w/ 14″ barrel, modified muzzle brake and vibration dampeners. Got a 3″ group offhand at 100′, 10 shots. That’s with the Crosman ultra mags .177, not my most accurate pellets but I have the most of them so will do the initial ton’o’shots to get close then zero with whatever I plan to use. I was pretty happy with that since we had the kids and my wife’s going” Are you done yet?” Lol

  16. After tossing around a few options, In still leaning towards the Octane .22, leaning… who am I kidding? I got the Octane, Kodiaks and a ten for ten bookmarked at the confirm order page! Just waiting for the next paycheck I can pillage!

  17. Hello all,late start,long day.For what its worth here’s my two cents on poi left\right impact.This is for you all that own the Airforce airguns.This is not a issue that would not detour me from purchasing another A.F. gun and you have to have a laser mounted on it to see what I’m saying to you.I use a green laser mounted on the top rail just ahead of the scope.This laser serves as a range finder for me.The laser is sighted in at 80 feet just as my scope is.This way when the green dot is on the cross hair I I’m at 80″.If the dot is lower,its under 80′ and if its over the cross hair then its beyond.This set up is for squirrel hunting plus shooting nighttime critters at close range.But again the laser should be mounted on top rail to work as I’m saying.I felt it was necessary to explain all that to get to this,so here is what I discovered last week.I had the laser on and I when to focus the scope and I noticed the cross hairs was moving or lets just say The hole scope was slightly moving from left or right? OK guess my rings could be loose? nope,then I change to a one piece mount thinking the hole time is the frame weak at the rail?Anyway the one piece mount was put on and still the scope would move BUT FLEX back into the same poi position.Now from time to time this scope will move its poi when just left alone from time to time.I know gun with out floated barrels will do this but a A.F. airgun is all metal.Having said all this I would still but a brand new one These are great gun and have giving me many pleasures and hundreds of pounds of meat on my table and for others to.So I say to A.F. ‘We re’s the beef?? The frame has a week area and needs more beef on the rail and around the trigger. A few ounces of metal in the weak places would stop any left right movement.I also understand the cost issue.Again most of the time the poi just will flex back but not always due to weather change or what ever.So before a hunt I will Shoot a round or two just for good measures to be sure of poi.Hope this all made seance to you I don’t claim to be good a this typing thing.I just a ageing hillbilly with a dial up computer that loves airguns and reading this blog.

    • steve
      I like that trick with the laser for determining your range. I put lasers on my daughters guns when they were learning to shoot. That way I could see what they were seeing through the scope.

      I never thought about using the laser like your saying. For range finding. That would be a quick way to see your hold over or under. Especially when shooting up into a tree or down a hill. You would then see the laser dot at the spot you should aim. Match it to the mil dot or half mil dot on your hold over or under on your scope reticle and your point of impact should be on the money.

      If it works out the way I think. I believe that you just made me a better shooter. I will try it this weekend and I cant wait to see what happens.

      • And for any body that reads and asks what do I mean hold under. Yes if I shoot at a squirrel up in a tree I have to aim low or hold under. If I aim dead on at he squirrel I will shoot right over the top of it everytime.
        Same if I shoot down hill. I will have to shoot low or hold under. Again if I aim dead on at the squirrel I will shoot right over it again.

        So If steves laser idea works I should know where the pellets going to hit at different distances and elevations. Well of course the feet per second and weight of the pellet will change that to some extent. But it should put me close to where I need to aim. I think I’m going to like it.

        • Don’t think the laser will help.

          The laser will (using his set-up) show the distance to target as 80 feet (say).

          Problem is, the HORIZONTAL distance determines the amount of gravitational drop, and for any shot up or down, the horizontal component will be LESS then the laser/sight intersection point.

          A somewhat extreme example (or maybe not extreme enough)…

          Sight/laser intersect at 80units (feet or yards). Squiddle is 48 units above shooter. If the sight/laser co-align on the squiddle, then the the squiddle is only 64 units away on the horizontal — and your sight plane should be based on a target 64 units away (if the 80 unit distance is SECOND zero, then the trajectory will be high at 64 unit distance, which is the real gravitational distance in play).

          • Wulfraed
            I want to try it first before I say anything. So this weekend I will try the laser with the scope to see if my idea works to show hold over or under. We will see. I will definnatly say what happpens either way if it works or not.

            • GF1,

              I have been using this phenomenon to demonstrate scope/laser use at the SHOT Show for over a decade. To do what I do, aim at a large wall that slants away from you and just move the scope/laser slowly along the wall. You will see the laser point move in relation to the crosshairs.


          • Wulfraed
            I want to try it first before I say anything. So this weekend I will try the laser with the scope to see if my idea works to show hold over or under. We will see. I will definnatly say what happens either way if it works or not.

          • BB and Wulfraed,

            BB I did your experiment and your absolutely correct. Now try my experiment and I think you will be surprised.

            Zero your scope and laser on a target as you normally would. Lets just say 30 yards. Now here is where things change. Now look through the scope and move slowly towards you looking at the ground in front of you. The closer you get to yourself the farther away the laser dot will be below your reticle zero.

            Now move back out slowly to your zero point at the 30 yard target. Your dot will be lined back up with your reticle.

            Now go off to the side of the target at the 30 yards and look at the ground with the scope and start moving slowly away from you. The dot from the laser will start moving up and farther away from the reticle the farther out you get from 30 yards.

            If you laser isn’t good enough to use out side then just tape a target on a wall in the house (your not going to shoot the gun so don’t worry about were you tape it). Hold the reticle on the bullseye and then zero your laser dot.

            As you move down the wall towards the floor the reticle and the dot will stay aligned with each other. Soon as your sights meet the floor and you get closer to you the dot will start going low. Now reverse it and go back to the target and go up the wall till you hit the ceiling. The dot will stay inline with the reticle until you start moving towards you. But guess what this time as I get closer to me the dot still moves down and farther away.

            So that means when you are on a flat horizontal plane the optics see differently. I’m guessing it has to do with the parallax. Try it for your self and tell my what you think.

              • BB
                I’m going to try it while shooting this time over the weekend and see how it effects my pellet point of impact in relation to the scope and laser. If it works out I’m going to be laser broke next pay day. :)

            • Well I tryed it outside. But had to wait till evening to try because I couldn’t see the green laser dot out past 30 yards because of the sun. I probably could of shot in the shade. But I waited.

              I took one shot straight and level at the target at 50 yards which is the scope and laser zero point. Then I took one shot in at 30 yards and shot with the scope reticle held low like the laser dot showed which was 1 mil dot on my scope. Then the last shot was at about 65 yards out and the laser showed a little over 1 mil dot high. So I took the reticle and aimed a little over 1 mil dot high and shot.

              All shots were pretty true when they hit the target. And it worked out best at 40 yards and in. When I wanted to make a shot in at 15 yards all I had to do was look how low the laser dot was below the target, see how many whole or half mil dots it was low. Then I held the reticle that many dots low and the pellet would hit on target every time.

              The only problem I see with it is that a real good laser would be needed so you can see better in the bright sun light. I don’t know if they make a laser that bright (I guess they probably do but I’m sure their not cheap). And the other thing is that from shooting my gun so much. I already knew what mil dot I should use for the different ranges.

              But it was kind of a cool test. And now my .25 Marauder has a laser and spotting light on it. And my .177 cal synthetic stock Marauder has just a laser on it now (no spotting light on it). And now to go out and do some more shooting. Looks like its going to be a nice day outside.

              • The LASER you can see in sunlight will be the LASER that causes near instant blindness.

                I have an older Wicked LASERs green model… Lacking a heat sink it is to be left on for less than 30 seconds, and then allowed some time to cool off. It WILL, however, burn through black electrical tape (and came with filter goggles to block the frequency of light).

                Pointers run <5mW; this Wicked model is around <125mW.

  18. B.B.,
    You mentioned in a few previous articles that you never seen a scope that is perfectly (optically) centered. The closest you seen was a scope whose reticle moved about 1/8″ when rotated 360 degrees on a target at 20 yards; most scopes can get only within 3/8″.

    In general, would you say a more expensive scope (leupold or Bushnell Elite) is more optically centered than a cheaper scope?

    • Joe,

      Yes, I would agree to that, in general. But then somebody wants to know the exact price point at which scopes become better, and I can’t tell them that. I have seen UTG scopes selling for $225 that were better than Leupold scopes selling for $350. So my general statement is pretty vague.


    • Strange… For some reason I’ve never considered Bushnell an “expensive/high-quality” scope… Middling utility hunter scope maybe — but old (60-70s) Weaver’s felt higher quality to me. Leupold was the high-end for American-made scopes back then.

  19. I have often wondered what would happen if a rifle could be shot vertically (straight down) with different pellets. So the different rifle zero would be over the average of some number of different types of pellets which have been fired vertically.

    Now the pellets are fired horizontally. The POI will be low compared to POA.

    I’d also expect a much larger variation in POI when pellets are fired horizontally (both in vertical and horizontal distance).

    The point is that the difference between the POA and the POI for a pellet would give some sort of characterization of the aerodynamics of the pellet’s flight.

  20. Hello B.B. Good morning !

    Thank you for the article, it was exactly what I thought about!
    I fixed the problem last week doing the follow:

    1st – I got a caliper and measured the distance between the center of dovetails to each side of the mounts and I found they are not exactly centered with the bore/dovetail. Thus I went to a tooling factory and asked them to file the bigger side in order to make the mounts centered with dovetail when mounted over the rifle.

    2nd- I optically centered my scope using the mirror approach.

    3rd – I’ve assembled back the scope/mounts on the rifle, and shot at 12m.

    4th – For my surprise the POI was almost deadly at POA, and then I made some fine corrections changing the front mount with rear mount and/or changing the clamping side left to right. I also used one sheet of photographic film to fix the elevation.

    5th- After done the best I could with shimming I adjusted the turrets, no more than 5 clicks was enough to zero in the scope at 12m.

    6th – Then I shot at 35m and the POI was aligned with the POA 3cm above. Then I shot at 55m and the POI was aligned with POA and 2cm below.

    Now I am very happy !
    Thank you for help me with my thoughts :)

    Have a nice week and sorry for my bad English.



Leave a Reply