Why does my rifle shift its aim point? – Part 3

by B.B. Pelletier

Part 1
Part 2

Okay, today I’d like to finish this report. There’s a lot to cover, but I won’t go into detail like I did in the first two parts. Remember, the complaint was a point-of-impact shift from one day to another AND a shift in the middle of a shot string. Those two separate reasons could each cause a POI shift, but if BOTH of them are happening, it can only be caused by parallax or a broken scope or loose mount. I think Hegshen is careful enough to catch the bad scope and loose mount, so I had to go with parallax. But there are other causes of impact shift.

Cause 1. Positioning of the cocking knob
This is an AirForce quirk, because they have a cocking knob that must be rotated into one of two locking notches on either side of the receiver. Fail to do it, and the bolt will move during firing, causing drastic changes in the amount of compressed air that gets behind the pellet. That will cause inaccuracy and POI shifts. When I was at AirForce and answered phone calls about accuracy problems, this was one of the main causes of those problems. AirForce made a video about how to operate the rifle and put the instructions in the owner’s manual. If a shooter didn’t pay attention to those instructions, they often had accuracy problems. Some guys went to the extreme of looping a rubber band around the bolt handle and the gun, so the handle would always be pulled into the notch when not being used to cock and load the gun.

 

The cocking knob of an AirForce rifle has to be rotated into one of the two notches before firing.
Cause 2. Elevation adjusted too high
When the scope’s internal elevation is adjusted up beyond a certain point, the erector tube return spring relaxes to the point that the tube can move at will. This will cause a wandering POI or sometimes a jump from one POI to another. This is a universal problem with scopes and the principal reason I recommend B-Square adjustable mounts. You can usually feel when the spring tension relaxes, because the clicks become softer and mushier. Avoid this region of adjustment if you want accuracy and a consistent POI.

Cause 3. Temperature change
A large change in outside temperature will change how the lenses in your scope are aligned. This is usually a cause for a POI shift when more than a day lapses between shooting, but I once saw it happen in the middle of a field target match, when a thunderstorm dropped the temperature by more than 20 degrees in a short time. The only solution is to re-zero the scope.

Cause 4. Shooting at different ranges
I tried to make this point in Part 2, but if that wasn’t positive enough, I’m saying it again. Shooting at different ranges affects the zero of your scope. I’m not talking about elevation. Everyone knows pellets don’t fly straight and will print at different places depending on the range. But if the scope is not aligned with the bore of the airgun horizontally, it will shoot to one side close up and to the other side far away. You will never see this after zeroing, because you will have aligned the scope and pellet flight path at one specific range. But, change ranges, and the POI will shift to the side. When the scope is optically aligned with the bore, this won’t happen.

That being said, this one isn’t a big problem for most scope users. When you miss your POI on a deer, you’re still within the kill zone and no one is the wiser. It’s only when you’re looking for half-pellet diameter accuracy and get slapped in the face with a one-inch shift that you pay attention.

Cause 5. Cant
Cant means tilting the scope away from the plane at which it was sighted-in. This will throw the pellet to the side and also down. I did a cant test that demonstrated a pellet movement of as much as 6 inches at 50 yards when cant is involved. For that test, the cant was a measured 20 degrees in each direction, something that would never happen in the real world. But a 3-degree cant is possible for some people on some terrain that doesn’t give good cues as to level. That can throw you off by an inch at 50 yards.

Cause 6. Sidewind
A sidewind will blow your pellet in the direction the wind is blowing, and also either up of down, depending on the wind direction and the direction of spin imparted by your barrel. This is caused by a phenomenon known as precession. A gyroscope (your spinning pellet) that’s pushed in a certain direction will move at 90 degrees to the force. The movement will always be in the direction of spin. If the gyro is spinning in a righthand direction and you push it to the right, it doesn’t go right…it goes down. Push from the left and the same gyro doesn’t go left…it goes up. The gyroscope is touching the earth at its point where a lot of friction holds it fast. But a pellet flying through the air has no friction holding it in place, so it tends to go with the flow. Therefore, the sideways movement is usually larger than the upward or downward movement.

 

Precession causes a gyroscope to move 90 degrees to the direction of force in the direction of spin.
Cause 7. Changing the scope’s power
When the power is increased or decreased on some variable scopes, the point of impact can change. Apparently, it depends on how the lenses are arranged inside the scope. This used to be a common problem, but a lot of scope manufacturers have designed their scopes so it doesn’t happen as much anymore. I shoot a lot of Leapers scopes, and I’ve never seen a POI change when the power was adjusted on one of them.

Those are all the causes for POI shift I can think of. If you know of one I’ve overlooked, please respond.

49 thoughts on “Why does my rifle shift its aim point? – Part 3

  1. Considering all the problems shooters face, it’s a wonder they can even hit water falling out of a boat. I assume experience and practice plays a good part of this, right? I appreciate the knowledge though.


  2. Stop it BB your scaring people! lol too much to worry about! I re-zero my scope every month on most guns! I just like to know things are how they should be. Next time i have a problem ill refer back to this post (im bound to have problems).

    I have not had any problems with temperature in my experience. I go from 70 to 30 degrees f all the time between indoor and outdoor shooting. If the temperature had an affect (i am sure it does on some scopes), i would have a big problem.

    -sumo





  3. BB,

    you mentioned the other day you were working with engineers on a new mount for RWS. I recently stripped out the tilt adjustment screws on my c-mount (one of the recent B-square editions) because the set screw that holds each one of them in place boogered up the threads on the tilt adjustment screws and when i tried to back them out, they ate that aluminum up. maybe this new mount can be all steel? just a thought.

    jeremy



  4. My experience has been that it is not the “gun” but the gun/scope that can cause a POI shift.

    My Steyr LG100/Leupold Competition would shift significantly with temperature. I found that following a suggestion by Larry Durham to loosen one of the scope mounts just enough so that it could slide with temperature-caused expansion/contraction solved the problem. A one-piece mount might be another solution but with my current BKL mounts, this is working very well.


  5. Thanks for the very informative blog.

    What would cause a change of POI with iron sights? Will temperature also cause this, if so how much change in temperature?

    I have a Daisy 853 and I need to re-sight often.

    Perhaps, you can do a Part 4 to this report discussing iron sights.

    Thanks again.
    Willie





  6. Willie,

    With open sights I don’t know if most people see with enough precision for a temp shift to matter, but with a diopter sight they would. But just carrying the gun around will also cause the sights to move, so it would be difficult to test. Not impossible, just painstaking.

    B.B.


  7. B.B.

    The only B-square mounts that I can find on the PA site that are labeled adjustable are the two piece sets. Can I assume that the one-piece m?

    It’s nice to see all the things a pro would think about. I’ll take it as a challenge to see if can detect any of these things.

    Matt61


  8. sumo-

    I remember you mentioning books on rifle shooting that turned you around, recommended by some incredible shot who could hit targets at 700 yards. Do you have any titles you could pass on?

    Also, since you’re a knife collector, I was wondering about your method for keeping knives sharp. I first got interested in this after buying a Cold Steel Voyager folder which was the sharpest knife I had ever encountered–shaved the hair clean off my forearm with minimal pressure.

    Matt61


  9. BB, I’ve thought of something. This is a question I’ve referred and asked about a few times. I have never recieved a full explaination on it, though, I never really asked it in a way that was easily understandable. Anyway, since I think more of hunting rather than plinking applications, I’ve often asked about “cold rifle” firing characteristics: that first shot you mentioned being off aim, but other shots being better as the rifle “warms up”. I am planning to carefully expiriment on this and explain my proceedures over the next few months (allowing weather change to add in it’s effects on multi-shot plinking), so as to maybe dope out HOW it is affected by being a single “cold” shot or several good shots from a “warmed up” rifle. I’ll let you know when the page is available. JP



  10. JP,

    Here is a test you should do. Record the velocity of the first shot from a gun, then the velocity of the succeeding shots.

    I’ll bet the first shots will be as regular as the successive shots are – PCP and springer.

    B.B.


  11. Matt61 -

    There are a lot of good books on marksmanship, but if you are mainly interested in extreme ranges, you might want to read “The Rifle Shooter” by G. David Tubb. David has won more rifle competitions than anyone else in history. Eleven NRA national titles, 30 national NRA silhouette rifle championship titles and six NRA long range rifle championships. I’m really grateful that someone as talented as David is willing to share their secrets with the rest of us. He also has an interesting DVD called “The One Mile Shot”. David has his own website at http://www.davidtubb.com. Lots of really cool stuff there. Hope this helps.

    - Jim in KS


  12. matt61,

    if you want to keep a knife sharp then don’t use it. lol

    I use different sharpeners for different knives. The best thing for kitchen knives is the “crock stick” but i find little use for it on other types of knives. With just a crock stick i can get all my kitchen knives to cut paper with great ease (no slicing, it just glides through).

    But for real knives….
    stay away from electric sharpeners. trust me on that! I use a DMT duosharp for most knives (not refining past the “extra fine” 1200 grit). It cuts MUCH faster than a traditional water-stone BUT i would use a stone to go finer. don’t get the “extra extra fine” DMT stone.

    Basically, i would get get DMT stones from extra coarse to extra fine and then a 4000 and 8000 grit water-stone (made by norton). Don’t bother with the Norton’s unless you need your knife “surgical sharp”. After the 8000 grit you can cut a piece of paper like its not there. The edge is to darn glass smooth that it catches nothing and cuts everything.

    I know what your thinking – holding an angle is not hard. I have a cold steel ti lite and YEA they are sharp!

    -sumo


  13. Hi! Vince,
    (off topic)
    Did I thank you for the o rings you sent me? well, many thanks.
    I came across that you bought a 34 when you already have a 48. Like you I have the Phantom and I just got a 34 couple of weeks ago. Tell me, I like the 48 too, tell me I should get one too . . . .Thanks

    Henry (I’m the other henry)



  14. Leatherman does put a great edge on their tools. They are sharper than gerber and buck knives, that doesn’t say too much though. I would say leatherman puts about as sharp an edge as Benchmade does, and that says a bunch. I just a never used benchmade automatic (154cm steel) and a brand new letherman charge tti (s30v!!!) to see. I call it a tie. as sharp as they are i still prefer the colt 45. lol

    -sumo, or henry if you prefer


  15. Hi BB
    I hate to mix blogs but i cant comment on Remembering When because i dont have a blogger name. I followed The windbüchse of Prostl story and enjoyed reading it. You mentioned you had some pictures of a replica rifle. I was wondering if you still plan to post those. I am very interested in what it looks like. Thanks

    Nate in Mass


  16. Speaking of knives, I can’t say how much I love my new leatherman wave. Sumo, your right, the blade on my last leatherman was sharp, but not for long.

    Kyle.



  17. Hi BB,

    Just got my new RWS 460 yesterday. Can’t seem to get enough droop compenstation out of shimming the Accushot one piece. I remember you saying your tested 460 had 21 inches !?!?!? of droop! Is that right?

    I optically centered my CP 3-9×40, shot at 17 yds, and could not even FIND the POI on an 16 inch high target, shooting for upper left corner. I eventually figured out that I had something like 14 inches of droop from optical center at 17 yds. Isn’t that pretty bad for an underlever?

    And that was with about 1mm of plastic shim at the rear already. I never could get the POI high enough, about 1 inch low was the best I could do. Now I’m looking at BKL 260-7 droop mounts, but my arithmetic indicates that I need .09 or more droop compensation at the rings, not .007. The BKL site says it’s .007 per inch correction, but per inch of what? inches between the rings or inches to target?

    Why on earth does RWS put out an underlever where the barrel is THAT much out of alignment with the receiver?

    At my wits end in Idaho,

    Jay


  18. This may seem like a newbie question, but how do you use the Ballistol spray on a barrel? I’ve been poring over the “Maintenance and Cleaning” section, and have found that the only possible usage is on a patch. Am I wrong?

    14 in Fla

    P.S
    Can you reuse a J-B coated brush after using it? I’ve always wondered that…



  19. Hi BB, I hope you don’t mind me asking an offtopic questions.

    Regarding the Gamo Shadow 1000. Is the .22 model a lot more noisy than the .177 ?

    Is the CFX significantly less noisy?

    Thanks.

    Jonno.


  20. B.B. Thanks for interpreting my butchered comment and filling me in. Must always preview…

    Jim in KS, thanks for the recommendation. That is exactly the sort of thing I want. I’m more than happy to make use of the experience of others, but I make it a point to learn from the best. That’s why I’m here. :)

    sumo, thanks for your info. I had bought the Razor Edge sharpening system produced by the winners of beard sharpening contests. It’s good but I can’t quite get their hair shaving results or the original edge of my Voyager folder. I sort of got intoxicated with its cutting abilities and ended up trying to cut through a steel computer cable which folded the edge over on itself. The Cold Steel people must have equipment like you mentioned which I will follow up. The Japanese 8000 water stone or 12000 if there is such a thing seems to be the ultimate. Someone said that a knife sharpened this way will cause the hairs to jump off your arm when they see it coming. Heh heh.

    I suppose this is pretty off topic but there is something about a really sharp and well-crafted knife that is reminiscent of a good airgun.

    Matt61


  21. The Condor and Talon also suffer from the frame flexion if shoulder to firmly. They need a light grip. This is proven without a doubt by using a laser on the gun.

    talonairgun.com


  22. BB,

    Pls excuse me for this rather off topic.

    I like the macho form of the AirForce guns, I wonder why the Condor and Talon are not used and winning trophies in FT competitions? I understand you reported them as highly accurate.

    Dave


  23. matt61,

    with my sharpest knives the hairs do seem to “pop” off. yes, there is a 12,000 but i have never gone up that high (something to look forward to). Cutting stuff with a immensely sharp knife is addictive.

    its a bit frivolous to go past 1200 but hey, theres bragging rights! I don’t think we are bothering anyone.

    -sumo



  24. Jay,

    If you can be patient, the new scope base should be out in another month or two. It will completely solve your problem.

    For now, shim under the rear of the mount base, not the ring. Use something about 0.040″ high.

    The BKL has nowhere near enough droop compensation for your gun. Save your money.

    B.B.



  25. Optical centering,

    Optical centering has nothing to do with distance. It’s not a part of sight-in. It’s a way of getting the reticle adjusted to the optical center of the scope, so you can zero the scope with an adjustable mount. When you do that, adjusting up and down doesn’t throw the pellet from one side to the other.

    The distance given in the optical-centering post is 20 yards. That gives more precision than if done closer. But if 10 yards is what you have, it will work. Your scale of correction will be smaller.

    B.B.



  26. Talon,

    I hadn’t thought about flex. Good call.

    Of course you don’t really mean frame flex, because the frame is rigid. The flex occurs at the coupling of the reservoir and the frame. Holding a Condor like a deer rifle will cause it to flex there and that would induce parallax with a scope.

    I hope Hegshen is still reading these comments, because I hadn’t thought of that one.

    B.B.


  27. Dave,

    The AirForce rifles are very accurate, but so are the rifles used for FT competition. And all of the AirForce rifles are too lightweight to compete. It takes at least 10 pounds to keep the rifle from bouncing around. My USFT weighs over 16 pounds!

    Also there is the matter of the trigger. When you try to shoot through a small hole without touching the sides, it helps to have a light, crisp trigger. The AirForce trigger is usually crisp, but it isn’t as light as shooters need for FT.

    B.B.


  28. Sumo,

    You aren’t bothering me. I, too, am a sharp knife fanatic. I even had a custom spearpoint bowie made to my own specifications for a rendezvous kit!

    My sharpest knife and also the easiest to sharpen is an Eye-brand German hunter from Solingen. It has surgical steel. A very close second place is my Case Hobo knife, also with a surgical steel blade.

    I have generally found carbon steel superior to stainless for sharpening. I have a Ka-Bar drop-point folder with a blade made of D-2 that was very sharp initially, but I can’t get it back to the original sharpness.

    I have a Gary Barnes custom folder that sharpens very well, but as it is made of 440C, it looses the edge pretty fast.

    I;m also fond of good automatics, and just picked up a large Hubertus stiletto at the SHOT Show.

    B.B.


  29. Shim under the mount base!!!What a concept! No more bent tubes! Think I’ll leave shims in the ring too, ’cause I just re-calculated and I have something like 20 inches of droop with no shims from optical center + 2″ high mount. (really only need 1.25″ mount height)

    I can’t wait for your mount!
    Thanks,

    Jay

    PS, finally had to go to the 460 to deal with the altitude problem, the 46 just wouldn’t cut it up here.


  30. I currently have no means of measuring the velocity, and my test will be with two different multi-pumps. My first task is finding which of my pellets work best in the second rifle. Anyway, I’ll keep posted. JP



  31. I just wanted to comment that today I recieved the Bushnell Banner 4-12×40 scope I ordered to replace the RWS300 one that was supplied with my 460 magnum and within 10 shots I noticed the scope was creeping back. This is using the RWS C mount, I went looking for some kind of double sided tape or something to try and keep it in place but then I found something called friction tape at ace hardware in the electrical section and have put some of that on the bottom half of the rings. Havent had a chance to put alot of shots through it yet but it seems like it will be significatly more stable. I also read that a product called scotchkote which is used for electrical stuff is also a good thing to put on the bottom half of the rings to stop scope creep.

    Just wanted to share that with anyone dealing with simlar problems. I think the friction tape is the way to go since its easy to put on, easy to take off. Also cheap. I also put blue 242 loctite on all the screws in the mount except the tiny ones that adjust windage. I’ll have to give it a good workout tomorrow and see how it all holds up but I feel good!

    The bushnell scope is also very nice. Cant comment on reliability but its compatible with use on airguns and is significatnly better than both the RWS300 scope and the one that came on the Gamo hunter extreme I had.


  32. BB,
    I have centerd my Leapers 3-9 X 50 scope and am in the process of installing B-square adjustable mounts on my 350. How close do I need to get the aim point (by adjusting the mounts only) to POI before I start adjusting the reticle positions?

    Thank You,
    Wayne


  33. Wayne,

    Get as close as possible on the vertical line. The horizontal line is meaningless, since you will be adjusting for elevation/

    At the worst, you should not be off the line by more than 1/8″ at 20 yards.

    B.B.


  34. Well. Scratch the Friction tape Idea. After just a few shots I noticed it was starting to move again. So, I decided to try the Scotchkote and that stuff did do the trick perfectly! I emailed Pyramyd air about it because I think it would be a good product to carry and reccomend people try on scopes that wont stay put in the rings.


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