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Post by sockethead on Nov 18, 2022 13:24:02 GMT -5
So I finally got my ML out to the desert for some long range shooting. It's a 45 cal, 20 twist, shooting 300 Aeromax bullets at 2400 FPS using Blackhorn. Scope is a Vortex PST 2.5-10x. My 100 yard groups have been hovering on either side of 1/2" and the scope is zeroed dead on at 100. I fired 4 groups at longer ranges:
#1 307 yards First shot was a bit of a flyer (I'm blaming the rickety bench I had), next 2 shots grouped about 1" apart. The group was a little high and 2" right. So I adjusted the BC in my calculator from .400 to .450 and moved the scope 3 clicks left.
#2 435 yards Grouped about 3", with the center of the group 3" high and right of bullseye. No measurable wind showing up on the Kestrel, although I did feel an occasional slight breeze coming from behind me and to the right. If anything the wind should be blowing my shots left, not right. Getting a little confused.
#3 435 yards After moving the scope 2 clicks left and 2 clicks down I fired another 3 shots. This time I got another 3" group that was pretty well centered.
#4 525 yards I adjusted my BC again to .475 in the calculator and dialed my elevation accordingly. I then put 3 shots into a 2-7/8" group, elevation was great, but it was 5" to the right of the bullseye! Why are my groups walking to the right?!
I have done a lot of rifle shooting in my life and never have I seen such dramatic movement of a group when increasing distance. I had set up a plumb line before firing the last group to ensure I wasn't canting the rifle. So then I thought maybe the scope is not tracking right. I set my target back at 175 yards, dialed the correct elevation, and fired a shot. Then I dialed up 10 MOA and used the reticle to hold 10 MOA low. The next shot was touching the first. So the scope is tracking perfectly.
Rifle is not being canted. Scope is tracking well. No wind to speak of. Coriolis should be minimal at these distances, and I was shooting straight West anyway. The only thing that remains on the list is spin drift. My calculator says it should only be about 2 clicks at 525 yards, but I'm getting a full 2 MOA of drift. At this point I am beyond confused. Any ideas?
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Post by nick50471 on Nov 18, 2022 15:14:27 GMT -5
Shooter is the issue
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Post by Kyle on Nov 18, 2022 15:53:18 GMT -5
So I finally got my ML out to the desert for some long range shooting. It's a 45 cal, 20 twist, shooting 300 Aeromax bullets at 2400 FPS using Blackhorn. Scope is a Vortex PST 2.5-10x. My 100 yard groups have been hovering on either side of 1/2" and the scope is zeroed dead on at 100. I fired 4 groups at longer ranges:
#1 307 yards First shot was a bit of a flyer (I'm blaming the rickety bench I had), next 2 shots grouped about 1" apart. The group was a little high and 2" right. So I adjusted the BC in my calculator from .400 to .450 and moved the scope 3 clicks left.
#2 435 yards Grouped about 3", with the center of the group 3" high and right of bullseye. No measurable wind showing up on the Kestrel, although I did feel an occasional slight breeze coming from behind me and to the right. If anything the wind should be blowing my shots left, not right. Getting a little confused.
#3 435 yards After moving the scope 2 clicks left and 2 clicks down I fired another 3 shots. This time I got another 3" group that was pretty well centered.
#4 525 yards I adjusted my BC again to .475 in the calculator and dialed my elevation accordingly. I then put 3 shots into a 2-7/8" group, elevation was great, but it was 5" to the right of the bullseye! Why are my groups walking to the right?!
I have done a lot of rifle shooting in my life and never have I seen such dramatic movement of a group when increasing distance. I had set up a plumb line before firing the last group to ensure I wasn't canting the rifle. So then I thought maybe the scope is not tracking right. I set my target back at 175 yards, dialed the correct elevation, and fired a shot. Then I dialed up 10 MOA and used the reticle to hold 10 MOA low. The next shot was touching the first. So the scope is tracking perfectly.
Rifle is not being canted. Scope is tracking well. No wind to speak of. Coriolis should be minimal at these distances, and I was shooting straight West anyway. The only thing that remains on the list is spin drift. My calculator says it should only be about 2 clicks at 525 yards, but I'm getting a full 2 MOA of drift. At this point I am beyond confused. Any ideas?
If the center axis of your action and center axis of your barrel are not one and the same, this could be the issue for left / right impact shift not explainable by normal spin drift. If the center axis of your scope is not mounted directly above the center axis of your correctly aligned action and barel, the same result can happen. |
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Post by joelmoney on Nov 18, 2022 16:05:04 GMT -5
Are you using a scope level? If not how are you maintaining the plumb bobbed crosshairs to target? As Kyle says gun crosshairs can be vertical without being vertical to bore axis. www.brownells.com/gunsmith-tools-supplies/sight-scope-installation-tools/scope-reticle-levelers/vertical-reticle-instrument-prod6097.aspxTo get consistent results I have learned here that: First— Level gun and optic with above listed level. Second—Level crosshairs with plumb bob while vertical level is on. Third— Tighten everything down. Fourth— While everything is level and plumb add a scope level to get everything right at range and field. By leveling off turrets, rings, or bases you can be starting off on the wrong foot. |
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Post by Kyle on Nov 18, 2022 19:27:16 GMT -5
If a 26” barrel, action and scope combination were out of alignment on an action by .005”, five thousandths of one inch, at the muzzle and that misalignment fell out at 3 o’clock or 9 o’clock:
At 100 yards there would be .69” of windage impact shift
200, 1.38”
300, 2.07”
400, 2.76”
500, 3.46”
This misalignment can only be corrected at one distance in terms of windage. In terms of windage only, not elevation - converging lines, line of sight and centerline of bore, will only intersect at one point,. Two parallel lines stay parallel and at that point only spin drift and environmental wind come into play.
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Post by sockethead on Nov 18, 2022 20:40:24 GMT -5
Thanks for the replies fellas. I'm not using a scope level, but I don't think that's the problem. In order to get a 2 MOA shift in point of impact I would have to cant the gun about 7 or 8 degrees. And to get a full MOA shift when moving from 435 to 525 yards it would have to be closer to a 30 degree cant. There is no way I'm canting my gun this much. A couple of degrees is possible, and that may contribute somewhat to the problem. I did consider this while at the range, so I set up a plumb line next to the target to check my bearings. Everything seemed to be spot on.
A misalignment in the action and barrel shouldn't cause this much drift either. My #3 group was centered on the bullseye, essentially giving me a 435 yard zero. I then dialed up for the 525 yard group and hit 5" right. In order to get that kind of shift I would need to have the scope mounted 24" to the left of my barrel. Even with the original 100 yard zero, the scope would need to be offset 2 inches from the bore's axis to explain a 2 MOA shift in POI at 525.
The shooter could also be the problem, but I can't figure out how. I'm shooting well under 1 MOA at every distance, so whatever I'm doing it's fairly consistent. Why would I start pulling my shots so precisely just because the distance changed? I did not shift my position, my rest, shooting angle, or anything else when shooting at the more distant targets, I just moved the target further away and used the same setup.
What kind of numbers are you guys seeing for spin drift with these bullets? My calculator says it should be 0.3 MOA at 525 yards.
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Post by ballistic on Nov 18, 2022 21:00:25 GMT -5
Agreed on what most have already said but I will add what spin drift can do to a bullet. Generally slower twist barrels have less spin drift than fast twist but that’s not always the case. I have seen 1-8 twist rate 30 cals produce less spin drift than 1-10 at 1000 yards and even less at longer distances. Other issues are scopes that are set up plumb to the crosshairs but will drift off course when the dials engaged. A simple ladder test at 100 yards where you dial up 15 moa and even more -on a plumb line can verify if the bubble level is witnessing with the dials - I highly recommend that you do this as even some high end optics can drift/lean right or left. If your off you can loosen the rings and the problem is resolved. I won’t ever win a Kentucky challenge event but rest assured - right hand twist barrels will yield spin drift to the right at distance. I purposely set my zeros on all of my guns too .65” to the left at 100 yards. This generally gives me close to a bullseye at 500 yards and will be 3-5” to the right at 1000 yards.
It was also mentioned that there was a tail wind that may have been blowing slightly to the right where the shooter was shooting. And it wasn’t clear what the wind was doing at the target. This could have added to the hits moving right.
Take good notes. Try the bubble and ladder verification on the scope dials. If those items are good - your setup might have a little more than normal spin drift and that’s perfectly ok. Adjust for it and the bullseye is yours.
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Post by buckeye68 on Nov 18, 2022 21:59:28 GMT -5
All good information here.
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Post by Richard on Nov 20, 2022 12:44:56 GMT -5
My first thought was the scope mounts were not in line with the center of the action. Also, an action whose face is not square to the centerline of said action.....along with/or the shoulder of the barrel not square to the centerline of the barrel.
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Post by ballistic on Nov 20, 2022 21:38:29 GMT -5
This pic is an example of a Sightron scope I set up on a plumb line at 100 yards. I dialed up 7 mils (25.25 moa) while aiming at the bullseye at the bottom. The group drifted 1 moa left. The solution was I put my rifle in a vice -loosened the scope rings-then tilted the crosshair to line up with the impacts going left. The bubble level was on the scope tube and was not re-leveled to match the actions level. I shot again on a similar target and this issue was resolved. This will (as tested) also line up offset cantilever scopes on AR15,s so this should correct a scope that’s mounted off the parallel axis to the bore. Have done this several times on rifles that have had cheaper components and a few that had the best money could buy. It doesn’t matter if the issue is in the action or the mounts or the rings or the scope - this should correct it.  |
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Post by mike on Nov 26, 2022 10:50:56 GMT -5
Wow, fantastic scientific shooting information, you guys are way over my head. haha With me doing the trigger pulling, my little CVA SPML is Minute-Of-Deer accurate out to about 250 yards. haha I have no place beyond 200 yards to practice/develop so my long-range skills will never be honed to perfection. I do enjoy reading about these issues and certainly know to whom I should direct my questions.
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Post by sockethead on Dec 2, 2022 14:35:33 GMT -5
Based on a comment from a different thread, I am adding a reply here to Ballistic. He called me out as being dismissive of people's comments on this thread, being a know-it-all, etc.
Ballistic, your comments on spin drift were helpful, so thank you. You also suggested checking the scope tracking to see if the turrets were part of the problem, but in my original post I had said I checked that. I had less than 1/4" of horizontal error over 10 MOA of vertical at 175 yards (well within the average group size). Turret tracking is not the problem, or at least it seems highly unlikely based on my test. I stated that I had "a lot of rifle shooting" experience merely to help others understand my level of competence, not to brag. I would expect a different set of answers had I said, "This is my first time ever shooting a rifle with a scope." I do apologize for being dismissive of your comment. I had ruled out scope tracking already, you said it could be scope tracking, which shows that you don't think I'm competent, so I didn't feel like replying.
Next time I go out I'll do some additional testing to try and rule things out more conclusively. A 2-shot test is not very conclusive, and it's very possible that I just pulled one of those shots by 2 MOA and I shot a 1-hole group by chance. I don't think it's likely, but to be fair I will repeat the test with more shots to confirm my scope is tracking properly. I will add a scope level, set up wind flags, and double-check everything for proper torque and alignment.
And I will add another thanks to all those who have replied here so far. I apologize if anyone felt that his/her comments were not appreciated.
I have learned quite a bit from reading this forum and I'm sure I'll continue to visit here and read the posts. But as far as posting things myself or replying to threads, it's really not worth trying to contribute anything. Too much work having to defend myself compared to what I get in return.
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Post by jeepeater on Dec 2, 2022 22:40:06 GMT -5
I’ve shot quite a bit of long range with fast, flat shooting centerfire. I know canting affects horizontal impact at distance. What I didn’t realize until I started shooting relatively heavy, somewhat slower smokeless ML’s is how much more canting affects them.
I noticed the 300 grain Pittman Aeromax at 3190 fps was “drifting” right on me. A few inches at 400 yards, then more at 600, and 8+ inches at 800 yards. I’d never used a scope level before, but that’s what closed it back up for me.
Most people don’t realize how much cant affects impact because they don’t shoot at distances where it really shows.
Your setup will be even more susceptible to horizontal impact shift from cant because of the slower 2400 fps velocity.
The more “hump” in the bullet’s trajectory, the more cant affects impact. Just a few degrees will make a pretty significant difference, add in spin drift, a slight amount of tracking error, and it’s easy to be off 5 inches at 500 yards.
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