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Why do shot groups move?

by B.B. Pelletier

I couldn’t fit the entire question into the title, so here it is:

Why do shot groups move from side to side when the scope setting doesn’t change?

This question does not include different pellets impacting at different places on a target at the same range, which is caused by the individual flight pattern of each type of pellet. However, I’ll address that question before moving on with the first one.

Today’s question was suggested by Jay in VA, who really wants to know why different pellets or bullets shoot to different places, with respect to left and right. Jay, that’s caused by how each pellet or bullet is stabilized. Back around the turn of the 20th century, Dr. F. W. Mann conducted an experiment in which he locked a barreled action in a 3,000-lb. vice (his “Shooting Gibralter” rest, a concrete pier sunk 40 inches below ground level and extending about 30 inches above, and fitted with a cast iron lockdown rifle rest) and then fired the “gun” at 100 yards in still air. After each shot, Dr. Mann rotated the barreled action 90 degrees, until he had shot it in a complete rotation. His lockdown mount was constructed to allow this rotation, so the barrel was never unlocked from the rest.

The barrel was locked down and could not move except to rotate along the axis of the bore. Still, it shot to four distinct places, each separate from the other. One was when the barrel was straight up, the next was with the barrel rotated 90 degrees to the right, the next was rotated 180 degrees to the right and the last was rotated 270 degrees to the right. That gave four distinctly different points of impact at 100 yards. Some Pope barrels shot groups smaller than an inch this way, but other barrels with less pedigree produced four-shot groups as large as 16 inches.

So, if the SAME barrel shooting the SAME bullets shoots to four distinctly different places, depending on how the barrel is rotated on its axis, how do you think it will do when different bullets are used? That’s right, different points of impact. It has to do with the projectile’s rotational stability and the bullets finding their own path (trajectory) once they’re free of the confining restraint of the barrel. We might like to think that bullets travel along a precise path, but it isn’t always true. Sometimes, they have to diverge to the path that’s dictated by their own inherent stability. Archers know this more than riflemen, because arrows have their own independent flight rules and the experienced archer learns what each arrow wants to do.

The real question
But that isn’t the question I want to address today. I want to talk about this: When a scope is sighted in at one distance, it will shoot to the left or right of the point of aim at a different distance. There are two main reasons for this.

Unstable flight
The first reason for this is the fact that some pellets spiral in flight. In other words, as they travel downrange they don’t remain on a straight line; rather, they travel in a spiral that goes in the same direction as the spin introduced by the rifling. A righthand-twist barrel can produce a pellet that flies with a right-hand spiral. You don’t have to take my word for this, you can watch it for yourself on You Tube. The man who made and narrates that video calls the spiral a wobble, but we’re talking about the same thing. He found it was caused by a dirty bore, but I’ve seen the same thing through the scope when using certain pellets that were not suited to a particular barrel. Clean or dirty barrels aside, those pellets always spiraled.

The video shows a pellet losing stability and spiraling from that loss, but a pellet can also be inherently unstable and always spiral. When that happens, the pellet will group at different points around the point of aim, depending on the distance the gun is from the target.

You don’t need a high-speed camera to see this. How do you tell if you have this problem? Simple, shoot three groups at three different ranges. Make the sight-in range the first target and the other two farther away. If they look like the ones shown here, your pellets are spiraling.

Gun sighted-in at 25 yards. Pellets are centered.

Same gun with same sight setting but shooting at 20 yards.

Same gun and same sight setting but at 30 yards. This pellet is spiraling.

Scope misalignment
A more fundamental cause of groups moving from left to right is caused by a slight scope misalignment, in relation to the bore. If the bore is pointed at 360 degrees and the optical axis of the scope is pointed at 5 degrees (that would be 5 degrees to the right of where the barrel is pointed), the shooter will adjust the reticle to bring together the point of impact and point of aim. Let’s say he does this at 25 yards, where the pellet hits the point of aim. If the scope is misaligned as I am describing, then at 20 yards it’ll group to the right of the point of aim, and at 30 yards it’ll group to the left of the point of aim. Where it hits depends on how the scope is misaligned, of course.

At 25 yards, the gun hits the point of aim.

At 20 yards, the pellet strikes low and to the right.

At 30 yards, the pellets strike high and to the left.

This type of “scope shift” will always go in the same direction. The farther out you shoot, the more to the left the groups will be; the closer you shoot, the more they’ll move to the right. They’ll always maintain this right-to-left relationship and will never reverse directions. That’s a diagnostic you can use if and when such a problem arises.

While the above two sets of targets look similar, they were created by two very different effects. In the first set, the pellet’s moving in a righthand spiral that’s increasing in size as the pellet moves farther from the muzzle. In the second set of targets, the pellets are shifting from the low right to a high left position and will keep moving in a straight line away from the aim point (to the left) as the distance to the target increases.

A spiraling pellet moves from one side of the target to the other and back again as the range increases.

A pellet shift caused by scope alignment always moves in the same direction — in this example, right to left.

Pellet shift diagnosis
Here’s how you figure out what the problem is.

Are the pellets moving from side to side and back again as the range increases? That’s a spiraling problem, and the dispersion (distance from the aim point) increases with the distance. The solution is to use different pellets or perhaps to clean the bore.

Are the pellets shifting in one direction and only as the range increases? That is a scope alignment problem, and the scope needs to be realigned to correct it.

Don’t be discouraged
This problem confounds shooters all the time, and it’s often the root cause of a misdiagnosed scope shift problem. Don’t be discouraged after reading this report. You can mount a scope accurately enough that you will never encounter this kind of problem.

Knowing that gives you power over these two common problems. If you know they exist, you can watch for them and make corrections when necessary.

author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

100 thoughts on “Why do shot groups move?”

  1. B.B.,
    Great blog today. Really perfect timing, as my range time just last weekend was consumed by multiple sight-ins and head scratchin. How does a ham fisted knuckle-dragger like myself realign a scope? Are you talking tearing the thing off and screwing it back down again? Or is there more science to it than that?

    • Hankmcrae,

      No, that’s what has to be done. You reverse the front ring, then the rear ring. Maybe you swap the front ring with the rear. That’s why 2-piece rings are a better choice than 1-piece.

      Adjustable rings are the final step.


    • Most scopes sit a little off center in the first place. Some can be quite a lot off center. It causes the same effect as canting the rifle.
      Ask yourself if it makes a crap. In a deer rifle, you can cant or have an offset scope to quite a degree and still have the POI land in the kill zone over quite a difference in ranges.
      So what if you really have to keep the “lefty-righty” to a minimum for small targets? A small offset in the scope or just a small amount of cant (or difference in cant) can make a lot of difference at extended distances when dealing with a need to keep the “lefty-righty” very tight.

      Let’s use an example..
      One scope mount configuration I had on my TSS left the scope mounted off center by a a good 1/16″. Holding the rifle vertical as possible (bubble balance ) the pellet will clearly leave the barrel offset to the scope, and swing over to the POI at the zero range (30 yds). Then it will continue it’s horizontal travel to the same offset distance on the opposite side at 60 yds. Does this make any difference to a starling? Not much at 60 yds.
      But what about the “corkscrew boogy”? That will drive you nuts because it never shoots where it should at anything but the zero distance.


    • How about some more information?

      Were you shooting different pellets at the same distance , or the same pellets at different distances?

      How much POI difference were you getting if shooting at different distances?

      Was the POI running in the same horizontal direction with different distances?

      Did the POI keep moving to illogical points with the same pellets at different distances?


  2. A little off topic, but I need help.

    I bought a Crosman 2260 a year or two ago, and put the scope base on it to use with a Leapers 3-9x scope. However, I found the comb was not high enough for accurate shooting, summer ran out, and I had to go back to school, so I put the gun in storage.

    I recently brought it back out, only to discover that the base receiver, the one with the iron sights, is missing. Are there any iron rear sights I can mount on the railed receiver that will line up with the front post?


  3. Optically centering a scope minimizes this effect.

    The shift of groups also happens at the same distance, with different ammo, until the bore is seasoned. Less recognizable with airguns but it does happen. With airguns,at 25 yards, when I’ve changed pellet types, within 50 shots (5 ten shot groups) the poi with some pellets can settle in at as much as a 1/2″ difference from where they started grouping (center to center). I know this isn’t directly related to the topic but in my view is an interesting tangent.


    • Swithching pellets or lubes clearly does this with some of my rifles too. Probably all of them, but not always to a degree at which it is noticeable.

      Worst thing to do when testing pellets and lubes is to shoot a group with each pellet or lube then move to the next. Don’t do this with rimfires either. You shoot both until you are sure that the POI and group size has stabilized before you flag that target and move on to the next….and repeat the whole process. THEN you do your target comparison.
      When finished, pick the best setup, do your final zero, and never switch the ammo configuration.
      Now, who will be the first to mention lot#s?



    • Kevin,

      Interesting phenomenon! I hadn’t thought of it because I haven’t seen it (or noticed it) before. But you shoot guns a lot more than I do. I’m always switching from one to another and don’t have time to get used to any one.


      • Thanks for the VERY detailed reply, as well as to those who responded yesterday. This is exactly what I was looking for. There is certainly a lot going on here – pellet/bullet instability, optical centering, etc. I suppose the condition of both the barrel and crown influence stability. I have personally seen POI shift more than an inch when sampling different pellets at only 10m. A good friend and avid shooter told me once that he’d found that a barrel needs a few “seasoning” shots before a new bullet settles on a POI. Jay

  4. Everyone,

    After reading today’s report I see that I explained how to test for a spiral pellet, but the graphic and target set shows a slightly different test. However, if you look at the graphic you will see that by shooting two targets farther away than the sight-in target you will see the effects of the spiral, too.

    I guess I should have shown four targets for the spiral test, which would have made the results very obvious.


  5. Perfect timing!
    Last week I ordered a Hawke 3-9 Airmax that arrived yesterday. This Saturday I’ll be heading to the range to sight it in on my Slavia.
    I’ll be printing this off and taking it with me.

  6. Good afternoon, folks! Been forever since I de-lurked.

    I was frustrated by what turned out to be scope misalignment, and I think this problem could be very common. I had never been precise enough in my shooting to notice it until B.B. and friends helped spiral my airgun hobby out of control. You know the story: from tin cans with K-Mart rifles to FT with adult precision rifles before you know what hit you. But as my equipment and technique improved, I became more confident that I was seeing this horizontal offset in POI at different ranges.

    It was a comment from Kevin that saved me. The problem was my misuse of scope rings that are designed for both 11mm and 3/8” dovetails. I’ve always used Leapers/UTG rings from PA. Turns out that, at least on my examples, they come out of the box adjusted for 3/8” dovetails. The rings have two “claws” that clamp into the dovetail: one claw is machined into the ring, and the one on the other side is the one that you tighten down. If you look closely at this movable piece, it can be flipped upside down, and has claws of different sizes on the top and bottom. You need to use the larger end on an 11mm dovetail. Once Kevin pointed this out, I flipped these pieces on my UTG rings, and my scope offset problems disappeared.

    I reckon this problem probably affects most folks who use these rings in 11mm dovetails. The rings I’ve purchased haven’t included any documentation on how this works. Then again, I could easily be the only guy thick-headed enough not to notice the adjustable dovetail thingy.

    And now to re-lurk again. I wonder how many there are like me, who religiously follow the blog and the comments, but rarely chime in…


    • Genghis Jan,

      I was thinking about shooting your Marurader with the wind blowing from left to right at the shooting bench, but the pellets being moved from right to left at the 50 yard yard line crazy thing to see. What is the FT schedule for this season At Damascus?


      • Hi, Bruce! Yep, that was a fun day of plinking. I wonder if B.B. would agree about the range at Damascus: “the wind is always left-to-right, even when it isn’t!” The remaining DIFTA schedule looks something like:

        Sat Jun 18
        Sat Jul 16 (MD State Championships)
        Sat Aug 13
        Sat Sep 17
        Sat Oct 1
        Sat Oct 15 (“fun” match)

        It’d be a blast if you had time to come to one (or all!) of the matches!

        Say, you know what would also be *really* fun? On Saturday, June 4, our chapter is hosting its annual Veterans Appreciation Day. This event is an absolute hoot! Free food; fascinating historic military displays; military weapons to shoot at the rifle range (Garands, M16’s, Browning 30cal, etc.). And honest-to-goodness live-fire WW-II flamethrower demos! Really. Oh, and we’ll have the airgun range open all day (closed only during the flamethrower demos!). I know you proudly served. If you’re not booked that day, maybe bring yourself and the family, do some plinking, and soak up some well-deserved appreciation? I’ll be there at least half the day, manning the airgun range open-house. (the military guns are limited to one-shot-at-a-time, so your Talon will be at least as much fun as the Browning!) http://www.damascusiwla.org/VetAppDay%202010.pdf


      • Thanks again, Kevin. I would never, ever have figured that out on my own.

        One of the top items on my airgunning to-do list: “find a way to overlap a visit to one of the Colorado relatives with one of Kevin’s airgun shoots!!!” (on the list right after “meet B.B. Pelletier”)


    • Thanks for the tip on the Leapers rings having two claws which are for different mount sizes. I have these rings on three guns. I never paid any attention to claw selection as I thought that both were the same. I also bought the from PA and installed them as they were in the package. There were no instructions that I found on how to install so I just used them as they came.

      I have been seeing and ignoring some horizontal shift in the scopes but just used kentucky windage to adjust for it.

  7. BB,I always like it when a video captures something like this.I have used the barrel rotaion/impact shift phenomenon to my advantage.I had a scope that has incredible quality optics,from Hakko Japan,that I wanted to mount on an old Daystate Harrier.I have had trouble with the quality of adjustable mounts.The Harrier has an excellent barrel,but initially pairing it with the scope it shot too far right….and ring changes didn’t help.I marked the top of the barrel,and then began the time consuming task of indexing the barrel until the POI was optimum with the scope.
    I haven’t said much about this scope: it’s a first year “Shepherd” scope,3x9x40 with 2 reticals.One retical in the first focal plane,one in the second.Each retical is adjustable with it’s own turrets.Here is where it gets crazy…..one retical is a crosshair,the other is a string of circles strung vertically.Are you ready for this? The string of circles is one of four,that can be changed via a second knob on the elevation turret! Only one string of circles is in view at one time,and they are used for rangefinding
    and positioned at the correct vertical holdover for different popular rounds/ BC’s.The scope doesn’t make coffee though.

    • BTW,I left out a little important info….the “stadia rings” do zoom when the scope is zoomed,and they represent 18″ at the different ranges regardless of magnification.I am only using it because it is super easy to zero,and because only one retical moves at a time you can see your corrections in relation to the other retical.In theory,this allows one shot zero from a rest.You simply aim at the target center and fire.Then you aim again,and move the second retical to the POI while on the rest with the other still on POA.That part is pretty cool….and the optics are excellent.

  8. Afternoon B.B.,

    I just got June’s American Rifleman. Check out the “I Have This Old Gun” which is the last page. When are we going to hear more from you about this month’s gun?


    • Bruce,

      I saw it. A nice old Ballard.

      I’ve been cranking hard on the Rogue for a 5,000-word feature in Shotgun News and I haven’t had the Ballard out lately. But I did receive the new nose punch for seating the bullet, so now my ammo will be better.

      Soon, I hope!


  9. B.B.,

    Let me throw another variable at you; namely, what happens to the point of impact (POI) when you start elevating the barrel. In my case, this is a very real issue as I do quite a bit of squirrel hunting with my airguns, and they have a maddening habit of hiding out in the tops of the tallest tree. Talk about a whole nother can of worms, adding elevation into the picture will definitely open one.

    The worst part is that it’s very hard to experimentally verify the shifting POI as you elevate the rifle because there are few ranges that will let you fire into the air willy nilly. Not to mention, that it’s awfully hard to hang targets at the required heights anyway. At least on level ground, you could build a quick range card of the POI shifts by simply firing groups at targets set up every 5 or 10 yards throughout your expected range. That’s a very difficult task to accomplish if you wanted to build such a range card for each 15 degrees of elevation you expect to shoot (up to 90).

    In addition, my experience has been that elevation shifts magnify any hold-sensitivity of the rifle for a number of reasons. Oh, and canting is much harder to correct at extreme elevations. It makes me want a PCP, I tell ya. Although, I am rehabilitating an old Crosman 120 that I picked up off of craigslist. Soon enough, I will be able to run some experiments on the effectiveness of spring-piston versus pneumatic for hunting in the West Tennessee squirrel woods 🙂

    • Bobby Nations,

      I’ve got a scope and green laser on my Talon SS. They’re both sighted in for 16 yards, which is all the room in my yard to shoot. You can really see the difference in POI when you compare the relationship of the green dot to the cross hairs at varying elevations and distances. The laser can range from 2 mill dots above or below the intersection of the cross hairs.

      It makes a very easy way of seeing the change in POI. Hope this gives you and idea that’ll help.


    • Let me throw another variable at you; namely, what happens to the point of impact (POI) when you start elevating the barrel. In my case, this is a very real issue as I do quite a bit of squirrel hunting with my airguns, and they have a maddening habit of hiding out in the tops of the tallest tree. Talk about a whole nother can of worms, adding elevation into the picture will definitely open one.

      You’ll get a vertical shift as gravity is now changing time-of-flight.

      Start with the common “horizontal” zeroing scheme… Gravity is only pulling the projectile down; the only significant effect on velocity is air drag.

      Now, for the extreme example, say you are shooting straight up at a similar distance… Now you have both air drag and gravity directly affecting the velocity of the projectile — it will take longer to traverse the distance.

      For something in between (say 45 deg up), part of the gravity pulling down on the projectile is retarding the velocity. Again it will take longer to traverse the distance, and in that longer time, gravity has more time to pull down on the projectile… It likely will hit low.

      Shooting down hill is the complex case… My mental impression is that, since gravity is now partly countering air drag, the time-of-flight will be less, and the impact may be higher as it hasn’t had as much real “down” applied… OTOH, the minor velocity increase may indicate the projectile doesn’t reach “zero crossing” and will be low, again…

      Hmmm, wonder if that trajectory program includes take-off angle changes…

      • Wulfareed,

        Most ballistic calculators will take into account elevation when running the solution, so you can chart a couple of scenarios (say 15 deg, 30 deg, 45 deg, 60 deg, etc.). I typically do this the night before heading out to the woods and remember general parameters to use in the next day’s hunt. It’s sort of like using Kentucky Windage but in the elevation plane. It’s not ideal, but I don’t usually have time to consult data books when trying to get off a shot. Heck, as fast as squirrels tend to move (especially in the springtime), it’s not unusual for me to miss out on shots while I go through my normal pre-shot routine, and that only takes about 3 seconds from the moment that I mount the gun.

        BTW, it’s counter-intuitive, but you have to aim below the target whether shooting uphill or downhill. And, the amount that you aim below is directly related to the size of the elevation angle. That is, the hold under amount increases as the elevation increases (or decreases). I see this all the time in the squirrel woods as well as in the deer woods when hunting from tree stands. It also matches what the ballistic calculators predict.

      • B.B.,

        Chuck told us about a SunOptics scope with a built in level. You tested on, but weren’t satisfied with the actual magnification verses the claimed magnification, that is, if my memory if working right now.

        I cannot speak for Chuck, but I really like mine. An internal level in a scope is a very good thing!


        PS Chuck are you still enjoying yours?

        • Mr B,
          Yes, I still use the SunOptic scope. It’s a 3-12×40 with a sidewheel AO knob. I use it around 9x. As a matter of fact, it’s what I used to produce that stacked pellets photo I posted about my IZH-61 the other day. I find the level inside the scope concept extremely useful and am very puzzled as to why they don’t exist in more scopes. Does someone hold a patent on that idea preventing its proliferation?

          As a cross test I bought an external level for my Talon SS. It attaches to the scope rail. Using the two ring scope mounts, there is ample room between them, under the scope, to fit the level to the rail. I wanted to see if there was any benefit to having the internal level vs the external. With the external level, I am able to very easily switch my attention to the level and back to the scope cross hairs by using one eye on the cross hairs and the other on the level. A little practice is needed but then it becomes second nature. Both eyes should be open anyway.

          The advantage goes to the internal level scope, however, since the attention/eye shift distance is minuscule compare to the external distance. Not only that, but the internal level is illuminated (along with the cross hairs) making it very easy to see the level out of the corner of your eye with no eye movement needed. Also, I found that even when the battery goes dead there is still enough light reflecting in the scope to see the bubble and get a consistent zero cant. I stacked those pellets with the battery removed.

          Now, keep in mind, I shoot only at 10m so the complaint that BB has about the test scope lacking on this particular model doesn’t apply to my use. If I were shooting at 50 yds it might be a short comming but I can’t really say that because I don’t shoot that far.

          I don’t know what the avaialbility is for these scopes, now, but the last time I looked they were out of stock everywhere I looked. I just wish so much Leapers/Centerpoint/Hawks would design one for around $100-$150.


      • B.B.,
        Do you mean, built into the scope, and visible through the optics? If so, who makes such a scope? Are there attachments for scopes that are useful? I used a bubble level for iron sights, but didn’t have such a thing for my scopes, so I just used reference markings on the target to approximate leveling.

          • BB

            Perhaps I may be so bold as to request that you revisit this scope? I have heard many good reviews about this scope lately, and it would be a good buy for the money if everything is on the up and up. I have considered buying one, but have held off. The internal bubble level is a great idea. If it is a dog, you can tell us why. And maybe the manufacturer will improve it.

    • B.B.,
      I don’t think this was mentioned, but using the shortest scope mount that will work with that scope and gun combo will keep cant error to a minimum. It will still be there, but just less of it. I try and keep the front bell just barely off the barrel or receiver.

  10. I’m surprised nobody has discussed adjustable rings yet. B.B. mentioned them right in the beginning. If I remember correctly, Kevin turned me on to the Burris Signatures in a comment quite some time ago.

    Interestingly enough, I posted this response last night to a ring lapping thread, just before reading today’s blog.

    I don’t know why more manufacturers haven’t jumped on gimbal inserts. I simply cannot say enough good things about the Burris Signatures, and they are not outrageously priced, considering how well they function.

    Of course, they won’t compensate for pellet precession due to a dirty barrel or poor pellet/bore compatibility.

    – Orin

    • Orin

      I have never tried the Burris mounts. I did recently break down and buy what I thought were the outrageously expensive Beeman (Sportsmatch) 1 piece adjustable mounts for my TX200. They are not the best design out there, seeing as how you must remove the mount from the rifle to make adjustments. But once they are set, with my optically centered scope, it is absolutely perfect. Can you put a price on perfection? Perhaps some bean counters can weigh in?

      The B-Square adjustable mounts have gimbals. I found them sorely lacking for a variety of reasons.

    • Orin,

      Don’t know why the burris signature rings with the pos align inserts aren’t more popular. They work flawlessly and are simple to allow windage and elevation adjustment without touching the scope turrets. Especially simple when compared to adjustable mounts.


  11. An interesting and deep subject to be sure. I know we went into this before, so I will just register my belief that pellets/projectiles do spiral but this spiral does not necessarily determine a helical flight path. They seem to me more likely to wobble within a downward-dropping trajectory.

    On this subject, I will also say that David Tubb claims that bullets will deviate about an inch to the right every 300 yards as a result of the Coriolis force.

    On another note, I found an explanation as to why a Lee-Enfield will supposedly tighten its groups as distance increases. I don’t believe that it is possible for MOA to decrease as range increases for reasons discussed elsewhere. But anyway, the theory is that the rear-locking lugs on the LE which allow it to cock on close somehow change the barrel harmonics and exert a stabilizing force at longer range. I’m forgetting some details now that made this sound a bit more plausible, but what I said seems to be accurate….

    As to picking your spot on the firing range, I had favored the extreme left side to avoid hot casings but there are fanatics who seem to wake up at 4am to secure it. However, another concern makes this irrelevant. I might take every caution to triple check my handloads and the shooting function of my guns, but I have no control over the guy next to me who might break all the rules of handloading and blow up his gun. My solution is to practice kneeling or prone where necessary which will allow me to hide behind these concrete structures that are used for shooting benches at my range. And this might also be a time to rationalize that expensive leather shooting jacket as well as a kevlar helmet. At $100, they’re not too expensive. Besides, wearing an army helmet and feeling a touch of anxiety with my surplus rifles would put me in the historical mood.


    • Matt61,
      The leather jacket definitely helped. The burn mark Mostly wiped clean. Had that been my face, or an eye, the damage could have been considerable.

    • Matt61, the rear locking lugs on the Lee Enfield don’t allow cock on close. Other rifles that cock on close have front lugs such as the Mauser 1893. The advantage of the rear lugs on the Lee Enfield is that the bolt head is easily changed to adjust the headspace.


    • RE: “Helical” path

      See another video at:

      Posted by Yrrah on the yellow. The first shot is repeated twice so there are only two shots not three. You can easily see that the “helical” path is large than the diameter of the pellet.

      The other part of this is terminology. We would need to define a couple of dozen terms to name all the things that are happening with the pellet.

      A pellet can precess and just curve. After the point that the curve changes to a helix, then predictability diminishes.

      The pellet can not only precess but it can also nutate.

      The “helical path” is not a perfect mathematical helix of course. It is just a curve that moves in an ever widening and irregular circular path.

      “Wobble” is as good a term as any when viewed form the tail. I think the term is particularly fitting if the pellet was fired from a smoothbore. From a smoothbore the pellet isn’t spinning so it can’t really precess or nutate.


      • There is something contradictory in this video. It has been stated on this blog that a pellet spins right (clockwise) because of the rifling and that the pellet spirals to the right (clockwise), yet, this video shows the pellet spirling to the left (counterclockwise). Did I read that right or am I just drinking Mr B’s coffee?

        • Chuck,

          You got it correct. I’m assuming that the barrel does indeed have a right hand twist. Almost all do…. That would make the pellets have a clockwise spin when viewed from the rear.

          The pellets are indeed spiraling counterclockwise.

          I think the explanation is this:

          * For bullets the center of pressure (COP) is in front of the center of gravity (COG). Thus bullets with a right hand twist precess clockwise as well.

          * For pellets the COP is behind the COG. That makes pellets precess counter-clockwise.

          A lot of the discussions on forums and blogs forgets that all projectiles are not the same. There are some significant differences between pellets and bullets.

          Hopes this helps,

          • Herb, that is a nice explanation, but everybody knows that Yrrah does his shooting Down Under. And everybody knows that projectiles spiral in the opposite direction in the Southern Hemisphere, just like the toilets and internet-forum login handles.


  12. B.B.

    I know its a bit early to ask or even wonder but when do you think the pogostick gun will be fixed, its got me so excited i wanna see how it groups and all.


    • Wojtek,

      I know how exciting this is for everybody. But remember, this design may not be complete. The gun may never have worked.

      As far as grouping and accuracy, this is a pre-war Diana 27 smoothbore, so figure 2 inches plus at 25 yards. Probably more like three. And given the power-robbing magazine, I figure it will be in the high 400s.

        • Wojtek,

          I think you will want to be first in line to get Dr. Beeman’s new book about the Girardoni repeating air rifle that has now been proven to be the rifle that Lewis & Clark carried on their expedition. Robert has shot his example many times and can give us reports on the power and number of shots, not to mention the incredible history of the very gun they carried. He and his wife donated this gun, now valued in the millions of dollars, to the U.S. Army War Museum, because they felt it should return home.


  13. Something I haven’t seen mentioned that could apply when changing pellets for the same distance… (And I probably wouldn’t have thought of it but for having printed Browning’s explanation for the BOSS system only a few weeks ago {I’ll summarize it later to save those unfamiliar with it from Google})

    Barrel vibrational modes…

    While going to a higher weight pellet typically results in lower velocity, and hence more gravitational drop time from muzzle to target… If the barrel tends to vibrate horizontally, the change in velocity could imply the projectile leaves the barrel when the muzzle is at a different point in the vibration pattern. {Actually, I suspect it vibrates in some sort of a Lissajous pattern!}

    Obviously, the more variation in muzzle velocity, among “identical” pellets, the larger the group size would become as each pellet leaves at a different position/angle/offset in the vibration pattern. So going to totally different velocity pellets can have a greater effect.

    The BOSS summary
    BOSS (Ballistic Optimizing Shooting System, as I recall) is a micrometer marked threaded muzzle weight on some Browning rifles (and I think some Winchesters also produced by FN at the time) which can be moved out or in on the barrel. The idea is that one adjusts the weight for their preferred load so that the bullet leaves the muzzle when it is at an extreme point in the vibration pattern. Yes, “extreme point” — at these points the muzzle angle is undergoing the slowest changes as it slows and stops moving outward and starts to “bounce back” inwards.

    When optimized for the load, minor velocity variations still exit in close to the same barrel position. In contrast, at the center of the vibration pattern (which would be the “at rest” barrel position), the vibrating barrel is moving fastest, so even small velocity changes could have projectiles going “anywhere”, creating a larger group size.

    • Most of the top smallbore shooters seem to use barrel turners which work very much like the BOSS Wulfraed described. They swear it matters at the upper reaches of skill.

      I think the forces involved in air rifle ballistics are too low for barrel vibrations to matter very much (if you are talking about 4.5mm pellets; big bore/high powered air guns are probably somewhere in between). Barrel vibrations can only account for a lateral/vertical shift in aim point. They won’t impart a spiral motion to the pellet in themselves.

      Edith: Nice looking avatar; do we get a better look at the lady soon?

      • Pete,I hate to disagree,but……I’m right now working on adjusting the H.O.T.S. on the Whiscombe JW80 fixed barrel.I can assure you that when it’s not in optimum adjustment,the groups are abysmal.This from an airgun with near zero recoil,so the vibration isn’t caused by the recoil.I’m not close to finished yet….but when I am I will share the extent of the results.

      • Respectfully disagree.

        This is not about the ballistics of a 4.5 caliber pellet but about barrel harmonics.

        The limbsaver de-resonator was an ugly accoutrement but interesting test a few years ago on airguns that I did. Not only tested as received but shaved about 1/4″ of meat off of one side to match the boss system. Not only did I shoot lots of groups rotating that gob of snot around a 360 degree axis but I moved it fore and aft. Curiousity kills cats and it almost killed me. Pellet groups certainly moved but didn’t tighten.

        I spent a lot of time with a maccari hot tuned R9 and a vortek brake. The groups tightened somewhat but either the scope shifted, the mount shifted, the stock screws came loose or my shooting changed from session to session since the groups would stay tight but would shift from one outing to another. I would try to tighten everything before messing with the vortek but at the end of that ordeal I came to the personal conclusion that I really didn’t need another variable in my airgun shooting that in the end made a very miniscule difference in performance.


      • Pete,

        That picture was published in a blog last year (on the old blog). I don’t remember which one, but it’s there in full size. I’m holding a Sharp CO2 carbine that I used for our field target wacky match. I loved that gun, which Tom promptly sold.


  14. I know I’m the most easily distracted person in the world so heres another question for the air gun experts. Where can i find the QB 78 pcp kit i looked on compessco and cant find it after watching their video how to install it.

    I know i butchered the name of the company I’m not sure how to spell it and the tab is already closed

  15. B.B.,
    Sorry I missed this yesterday, its a great blog.
    I remember a while back you talked about shooting in the morning or afternoon with a low sun angle at your back. I tried it then, and it does allow you to watch the pellet. You can actually see the pellet spiral, or not spiral by changing all the variables. If you can shoot a long distance it is easier to see it way down range.

    I think this also gets back to what you always preach about finding one pellet that your gun likes the best. It also shows why there are certain pellets that you always try, because they are proven winners. The CPs and JSBs are always good bets and if you look at their inherent balance, they are very nose heavy which helps to keep them stabilized in flight. A very bad pellet (balance-wise) is the Gamo Master Point. The skirt hollow runs almost up to the nose, and it has a small head and very pinched waist. That puts their balance point close to its center, lengthwise. Pellets need to be pretty nose heavy for the “badminton shape” to keep them stable at longer distances. I actually put a little nick on the skirt of some pellets for the sunlight test and it really made them do crazy things. But the right pellet in the right gun can give very repeatable results, even if there is a little spiral.

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