horizontal poi shift with distance change
I have seen some discussions, with inconclusive answers.
Vulcan 25, paired with a 4-12x Leupold scope borrowed from my powder burner.
Using 25Gr JSB, MZ 890fps, zeroed @ 25 yards.
With no wind, the poi@100yard will shift to the left about 3moa, fairly consistent.
Anyone has a solution? or should I just live with it.
All Replies
Yep, the Burris ring is next item on shopping list when budget allows. I will play with current set-up some more, also want to experiment with different muzzle velocity and its influence. That’s another can of worms.
The mount I have now is a one-piece quick-detach, supposed to be quite sturdy and overkill for pcp. But I can not turn the rings 180:
Tall target test and rotate scope:
I did some basic cleaning and re-install of rail, followed by my Tall Target test: After zeroed at 57 yard, I clicked up 45MOA, the new group is 26.75″ high, and 0.75″ to the right. Yes to the right, instead of the previous error to the left. So I rotated scope CCW, till poi lands on vertical line. However the cross hair now looks canted to me when I use natural hold of rifle. That being said, when I dial the turret to desired elevation and hold rifle canted, I could put poi on the spot between 20 – 60 yards without adjusting windage. But the un-natural canted holding position is a big annoyance that I can’t live with. So, got to try shimming the scope.
Background:
It’s a 30mm GG&G accu-cam quick release mount, and a Leupold scope I borrowed from AR. The system is repeatable to my satisfaction on AR, despite on and off, zero is staying good enough for plinking. When I change guns, I note the elevation and windage change from a reference point, so I could return to zero for each gun. For this reason I was reluctant to shim the scope rings.
Back to Vulcan, in an attempt to remove the stress on the barrel per Kris’ suggestion, I loosen all the fasteners around air-tube clamp, and re assembled them carefully. One thing worth noting is that my elevation shim created strain of barrel. I had a 1mm flat metal shim under the rear of the Vulcan rail. Because the piece I used is not an ideal wedge shaped shim, it creates upward stress on barrel: the barrel could not be turned by hand if the rail is tightened with the shim in place. Although I don’t think it causes my windage error, but for the remainder of the test I removed it to eliminate variables.
Starting from scratch I gently, very gently, lapped several passes on the scope tube contacting area, rings, and contacting area of rail-to clamp, with 1000 grid paper. Next I found and tuned scope to the optical center, mounted it on Vulcan. Without any adjustment, POI grouped to the right by 12MOA from the optical center.
I think 12MOA is a lot. It is possible my scope or mount is bent, that need to be proven next time in powder burning range. So for now, assuming my rail and scope are straight, I’ll treat it as the barrel is bent / pointing to the right by 12MOA.
The Vulcan rail screw holes have some left-right tolerance, so I used it to my favor, tried to reposition rail, though such position is difficult to repeat every time. Until I can not squeeze anymore adjustment, poi still have 2 MOA error to the right. That’s not too bad compared to 12MOA, and I can deal with that using turret adjustment.
Trajectory improved but I still see some right-to-left POI shift when I increase range. Tested a few shot at 100 yard, results all over the place due to wind at 13mph, and I definitely still see curveball through the scope from right to left. The symptom is still there and wind is not the only blame. I will need to do a better measurement on a cool and calm day, maybe the tall target test again.
Through this exercise I learnt that a canted-hold can cause trajectory that looks like a curve ball through scope. I put a level on the rail so I can see it when aiming, hope to confirm my holding. The Vulcan is still very accurate, repeatable, at 57yds it is putting pellets through one large hole if I do my part. For punching paper at same distance everyday, I would be perfectly happy with the result.
To be continued.
I think it correlates to your earlier suggestion of “shooting a square” to see if the 4 poi s can create a square vs diamond.
Lately I have not yet found a windless day to test it, but based on what I just saw, I am almost certain the scope needs to be rotated in the ring.
Yes that’s that’s right …… that guy’s mother is as old as the old canting test.
To get an offset of 3 MOA @ 100 yards is a lot for it to be 100% fixed by just a rotating of the scope in its rings, but worth a try ?
Some (yes they could be a little of each) of this small error seems more likely found in a slightly twisted frame, slightly offset pair of scope rings, slightly bent barrel (there’s no such thing as an absolutely straight barrel) or a bad/damaged crown.
There is an error on your gun that’s about .01″ from it’s front to it’s back, that’s about the thickness of 2 layers of beer can shims.
All in all the main thing it’s shooting constantly and the grouping is good, there should be a fix.
Keep us updated on this, I’d like to know Sir ….. thanks.
~ GReg
Greg, that’s great information.
I think it correlates to your earlier suggestion of “shooting a square” to see if the 4 poi s can create a square vs diamond.
Lately I have not yet found a windless day to test it, but based on what I just saw, I am almost certain the scope needs to be rotated in the ring.
~ GReg
The coolest old school method to check canting error ………. other than my jig/measuring tool above.
U need a large backer board and keep the gun level at all times when shooting.
Take 3 shots on board and this will be your POA for the shots below.
10 MOA / clicks up take 3 shots.
10 MOA / clicks left take 3 shots.
10 MOA / clicks down take 3 shots.
If the connected groups are a perfect square, no canting error.
If it looks like a parallelogram, u have canting error.
In the direction that it leans is the canting error direction also.
Assuming no wind of course.
~ Greg
ok i will write you down exactly what i had on my hw100.
the .22 was shooting 1cm right from the POA at 5m. of course it is NOT possible to to have a 1cm off scope horizontally on the dovetail rail. and NO it was not parallax.
it was shooting dead spot on at 25m.
I turned/indexed its barrel anticlockwise looking from the muzzle end 45degrees. this moved the poi at 25yards a lot 10cm or soo(cant remember exactly) too much to left from the POA. i brought the POI back back using the windage turret so moved the poi to right to have the POA=POI.
now when i shoot 5m or 25, there is only holdover.vertical difference.
make sure it is not cant (it has more effect at further distances than at close distances), the way that when the top of the scop is canted too much left, the POI willl be Left from the POA. or vice versa right/too much to right.
try to play with the scope rail or maybe the cylinder clamps, obviously you cant turn the barrel on a Vulcan because the TP is fixed.
when you are onto this target shooting level of accurizing/adjustment make sure your gun groups tight and the poi is not moving along the fill, otherwise you will be chasing your tail forever.
.
Also a note on the rail and the barrel of a BP when it’s not in or on the same plane.
In most BP designs the barrel goes into a fixed breech and can pivot out of there with it’s rail with it, twisted not parallel.
This is an over exaggeration top view of the problem I had with my R3M. The POI was to the left, and all I had to do is loosen up CCW the large barrel tensioning hex bolt till the POI was on Optical center within a click or so.
That sounds like what you’re experiencing with a curve ball, hook shots.
Dang, there isn’t any thing u could add to a gun that’s more complicated than an addition of a scope !
~ Greg
.
No …… to my question.
I usually Optically center all my scope before mounting and shim or adjust to zero the best I can to within a couple of clicks.
If it requires a windage adjustment of more than 1 MOA to zero, I’d suspect that the barrel and scope rail are not in the same plane.
If so I make use of the fact that the UTG rings are a bit off sided per set.
So I flip only one over, one eighty meaning ……. the affixing is switched to the other side.
Then I check again and see if there is and difference in the POI or I use the jug above to see if it’s better or worse.
Then it depends on the gun in question, if the problem can be resolve from any other adjustment by design to aid in a realignment.
If not, I’d wouldn’t force it to happen and make my adjustment in the rings.
HFT guys sometimes do this but I’ve notice this problem is more prevalently occurring in PB designs and less in long rifles ……
…. more info for yah here.
Setting Optical Centre at Optimum Range.
http://www.fekete-moro.hu/bfta-setup-manual/index-en.php#35
Lateral Misalignments of the Scope.
http://www.fekete-moro.hu/bfta-setup-manual/index-en.php#352
~ Greg
if you haven’t got already you can make yourself a night vision, and if you use a laser illumination, shooting from a hill/over the country you can see the trajectory beautifully.
16mm lens and some cctv camera with had II 1/2 inch chip, some 18650 battery 3pack, a bit of cutting and soldering of wires included. angle eye or similar dvr recorder. ir torch with a large fish eye style focusing lens in the front may do as well if you cant get a focusable laser to get you the extra range.
maybe you can record with only a strong focused torch only. the key is to have dark background/no obstacles only the pellet to reflect the light back. if you pull a shot, on camera the trajectory will look curved because the camera moves too.
also if you look up some close range ratting videos, majority of the footage have trajectories like the pellet starts its flight off center horizontally, and of course at the first or whatever zero range it meets the crosshairs. no I don’t know if all these guys have bent barrels, or it takes some distance to stabilize a pellet or all these guys have their scopes setup wrong or cameras off center, but they just ignore it. they all must have crossover then?
OK that’s a lot of information, I need to digest your suggestions and tinker with it.
@pippo, can’t turn the barrel because the port location is fixed.
@Greg, I see you painstakingly measured out the cant angle of the scope vs barrel, and shim it. Wouldn’t you achieve same effect by adjusting windage turret?
@kris, interesting thinking about the turret erector tube not centered in the tube…
I don’t have the data to prove it but my guts feeling is that my gun seems to be throwing a ) curve ball, a repeatable curve ball from right to left.
Assuming my scope is parallel with barrel. After pellet leaves the muzzle, regardless of bent barrel or cant scope, it’s path SHOULD fit in a vertical plane, I want my scope line of sight to be in the same vertical plane. Real world is not that simple so my scope maybe offset from the vertical plane, but no more than 0.1″. Anything more than that I can easily feel or measure it. Zeroed at 25 yard, the poi shift due to scope offset, is no more than 0.1″ left-right, at 100 yard, it should not error more than .4″. In my case it hits to left 3 inches. It makes me believe the gun is throwing out the pellets like a left/right curve ball, instead of conforming to a straight vertical rise/drop.
I also saw some articles suggesting to tune the pressure and hst to find a magic velocity and rotational speed. That’s a very extensive experiment…
dont forget the 2 screws on the scope rail mount are usually very tight, but if you undo them there is enough play there both in the front or the rear to adjust for windage and then you can have more play with the turrets, the erector tube is not necessary parallel with the housing so you can spend time messing around with the outer alignement it is no good when in the end you mess with the turrets too much.
maybe you should just undo the cylinder clamps and the screws on the trigger guard, shake the whole thing apart and put it together realigned. maybe there is bit of strain there, what if the front clamp was a bit rotated around the cylinder in comparision to the rear cylinder clamp.
i would just take the the whole frame with all screws apart, barrel loose, maybe it is too much tinkering for you heee? :biggrinn:
Never seen a tool like this, thanks for sharing :8:
Yes many barrels have this effect to some degree and this is what makes the difference between a good barrel and an outstanding barrel that you will never let go 😀
Yes it could be a bent barrel ………. but all of my Airguns had this effect to some degree, not sure if it was as much as 3 MOAs though.
This is my alignment gauge to check barrel and scope rail if there in/on the same plane. Pictures are shown with the stock on, measurements are taken with it off.
For a 3 MOA (3″ @ 100yds) error with an 18″ barrel from front to back would have a difference of about .015″ , the equivalent of 3 layers of beer can shims.
~ Greg
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I see, those rings are hard to shim for windage error ……
For my 2 Cricket their frames are not absolutely perfect with a very very slight twist, but I’m not going to try and straighten them.
So I’ve made use of the fact that cheapy UTG 2-piece mounts are not made perfectly.
Made use of this untrue parallel to my advantage by flipping, switching the pair front to back and even using one from another set.
All in all to help me get it to zero with an Optically Centered Scope within a couple clicks !
So, you’re presently 2 MOA off to the right of zero am I correct ?
If so with an Optically Centered Scope if you’re zero your windage at 100 yards at 50 (or X) yards you might still more or less be 2 MOA (4 or 8 clicks) off to the left, or should I’ve said alway be 2 MOA off at all ranges ?
If you’re half that, 1 MOA off at 50 I’d suspect a twist somewhere within your gun.
It’s such a small fudge factor that is why I said, might ……… and EVERY gun has it’s own fudge.
It all depends how finicky you are and how much error you can live with.
Also the heat/environment of the day can affect a perfectly zero set up by a couple of click anyway.
Some good reads info for yah here, like that’s not enough all ready ?
http://www.rimfirecentral.com/forums/showthread.php?t=486844
http://forum.accurateshooter.com/threads/optically-centering-scope.3765858/
Regards ~ GReg