Troubleshooting left/right zeroing

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bd2021

bd2021

Member
Nov 18, 2021
379
16
MN, United States
I am trying to troubleshoot a zeroing problem. It probably has a simple answer, but it is escaping me at the moment. Below are the essentials:

1. 10 yards, 120 gr: shot 5 rounds of 120 gr at 10 yards, 3/8" center to center grouping. Shots were centered 9 MOA down, 3 MOA left.
2. 10 yards, 120 gr: Corrected by 9 MOA up, 3 MOA right. Shot 6th dead center at 10 yards.
3. 30 yards, 120 gr: High flyer, moved reticle 3 MOA left, 19.5 MOA down. Somewhat curious about the 3 MOA left correction in light of above.
4. 30 yards, 120 gr: Shot 2 rounds, grouped well but was 3 MOA down.
5. 30 yards, 120 gr: corrected an additional 3 MOA up.
5. 30 yards, 120 gr: shot 1 round, dead center.
6. 30 yards, 145 gr: shot 1 round. Off by 3.75 MOA left and 3.75 MOA down. Did not adjust, went home puzzled about the leftward drift.

Puzzle 1: Why at 10 yards did I have to move up and right, but at 30 yards I then had to move left and down. One possible explanation is looking from the top down toward the barrel is not parallel with the barrel. I am wondering if I need better scope rings, or should lap or shim the scope rings.

Puzzle 2: I understand why the heavier bullet, which is going slower out of the regulated gun, will drop more than the lighter round. But I am having trouble coming up with an explanation as to why it is moving left. One possible theory is the barrel is flexing (I am not sure if that happens in air guns) and leaving the barrel at different points as the barrel oscillates left to right, based on mass of bullet.

Note: Benjamin bulldog with:
o Huma regulated to (I think): 2700 psi (May be 3100 psi, gauges are off)
o Pitbull barrel support
o Power port tune
o Power spring
o High performance breech o-rings
o MTC Viper PRO Tactical 3-18x50
o UTG PRO 30mm Medium Profile P.O.I Picatinny Riflescope Rings
o Crosman 20 MOA Tactical Weaver Rail

I am wondering if I should lap, and/or replace the rings/rail with a one piece unit?

If anybody has better explanations or suggestions, please let me know.
 
Bd2021,

Puzzle 1:

Screenshot_2019-07-04-23-06-44.1638049506.png


Puzzle 2: see above AND 

Maybe research "barrel harmonics". Most barrels move in 2 dimensions(up/down, left/right) when fired. Depending on the timing when the projectile leaves the tip of the muzzle, the tip of barrel can be moving (slightly) in any combination of these 4 directions. Different weights and/or sizes and/or speeds can affect this timing. That is why "barrel tuners"(long used by the powder burning community) are becoming popular in airgun.

Good luck
 
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Hello Just read your post. Please do not take this the wrong way I'm just trying to figure a few things out to maybe help resolve this problem. First what type of rest are you using(front and rear) also what are your typical groups at 30 yds because 3 moa at 30yds is only about an inch. From everything you said I would not be that concerned. I would make sure the gun is is on a solid rest and shoot 3 rd groups before adjusting zero. Also go on one of the ballistic pages (I like Americans Airguns trajectory calculator) and get some raw data as to where the gun should be printing and see how it compares to your true data. You can also do a box drill to ensure the scope is correctly mounted and tracking as it should. As for the 145 gr every projectile will behave differently Once again please do not take any of these suggestions the wrong way I am not trying to be anything but helpful, Hope this helps Eric
 
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for one, id never mix rounds, settle on the one you want and stick to those and zero ... and what scope? .. going off an assumption, but if its a cheaper one they tend to settle after each adjustment, and an adjustment on one turret can affect the tube both ways .. once its close with ONE pellet of choice just put a clip or two through it between each adjustment and move the group average .. dont chase it too hard .. the gun itself isnt a tack driver, but at '20' it should do under an inch for sure ..
 
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Hi Ghostranger169,

I think your tip on barrel harmonics may be at play for puzzle 2. I'll look up cant and see if it is also playing a role in puzzle 1.

Hi Eric and dizzums,

I am prone, using for the front UTG Super Duty Bi-pod and my armpit for the rear.

I am zeroing the 120 gr, only shot the 145 gr to see what it did, as my supply of the 145 gr is short. I want to see what sort of adjustment I need if I have to put down a coyote.

I am using the 120 gr, which I have a lot of, for breaking in the regulator.

The 3/8" center to center at 10 yards was a 5 shot group, and works out to be about 1.8 MOA by my calculations. At 30 yards I seem to be gettting about 1.4 MOA, or 7/8". Not bad, but I'd rather have 3/4" at 40 or 45 yards, which could very well be unrealistic.

I'm using a hand pump for now for the exercise, but it does limit the number of shots I take per day, with cooling breaks and rest breaks.

I'll look up the "box drill" and take a look at americans airguns trajectory calculator. Thanks for the tips.

Hi the scope is MTC Viper PRO Tactical 3-18x50. I am thinking about switching to a fixed 4x or possibly 7x scope, or maybe even sights.

Even so, I'd rather put a coyote down quick with an accurate shot. I was hoping the pitbull barrel supports would steer the performance more toward a tack driver. As you imply, dizzums, there is such a thing as good enough, but if in a controlled environment I can get it to be almost a tack driver, then I won't have anybody to blame but myself, and it will cause me to take more ethical shots.
 
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