Help diagnosing low shots at 600yds

[VA_XTC_Shooter]

I just wrapped up at the Creedmoor Cup at Butner this week shooting service rifle. I had 3 very odd shots at 600 yds.
Load: LC 16 brass twice fired, Berger 75 VLD, Federal 205 MAR primer, with 23.3 gr H4895
Barrel is a Krieger with about 700 rds on it.

I’ve shot about 300 rds of this load over the past two months with no issues.

So on Day 2 I was shooting at 600 and was on a run of 10s and Xs, then a miss at 6:00. Shot 5 more 10s and Xs and then a 5 at 6:00.
Went down to look at target backer to verify those shots and they were there.

My initial thought was a low powder charge. Would have to be about 19grs instead of 23.2 to go from 2850 FPS to about 2200 fps that would match up with a shot 31” lower than the rest.
So that evening I weighed each remaining round. All rounds were the same weight, within 2 grs (193.8-195.4 gr). So I assumed those two must’ve been odd balls and went into the next day confident my 600 ammo would perform.
Day 3, first sighted at 600 was another 5 at 6:00! The next 23 shots were back on call and I ended who with a 197-9.

Since all cartridges weighed the same, I’m ruling out a lower powder charge.

So what else would cause this? Bullet holes in card board looked normal so no signs of bullet tumbling or falling apart. I need to figure it out, can’t be going to a match with no confidence in my ammo.

 

[JFrank]

I wonder about the possibility of a primer issue

 

[243winxb]

Neck tension? Bullet moving on loading? Feeding by magazine or single loading?
http://www.exteriorballistics.com/reloadbasics/gasgunreload.cfm

So bullet impact was about 30" low?

 

[VA_XTC_Shooter]

I wonder about the possibility of a primer issue

Care to expand on that?

 

[VA_XTC_Shooter]

Neck tension? Bullet moving on loading? Feeding by magazine or single loading?
http://www.exteriorballistics.com/reloadbasics/gasgunreload.cfm

So bullet impact was about 30" low?

Neck tension is about .003 via measuring neck diameter before and after bullet seating.

Cartridge is single loaded via sled and releasing bolt catch to load in chamber, standard practice in high power with the AR.

 

[JFrank]

Care to expand on that?

Perhaps you got a couple that were not seated fully or perhaps just weak, just a thought

 

[expiper]

I suspect neck tension issues,,,when seating watch for one every now and then with almost no feel ,,,if you are single loading and fireing ,,,seperate the "bad" ones and when home look for anomilies like cracks in neck etc,,,,Roger

 

[TAJ45]

Gas Gun....... no chance the low shot brass was segregated I'm guessing.

 

[243winxb]

LC 16

Could there be a difference in case volume/weight? Markings should be the same.

Some US GI cartridge cases are run on SCAMP (Small Caliber Ammunition Modernization Program) lines. This is HIGH speed cartridge case making equipment. Several lines of SCAMP machinery may be used to run cases thru out the year, or on the same lot/year of ammo. Those dots identify which SCAMP line.

 

[akajun]

I doubt it’s neck tension that is causing a 30” drop in a service rifle. I’ve shot split neck rounds with good rounds in practice and never noticed a difference that ever cost me a point. There’s something else going on,
I’d start checking the rifle, make sure the gas tube is not touching something , barrel nut, scope mount tight etc. carbon build up around the gas tube hole in the upper and barrel nut can cause wonky shots.
It may be something with your loads but I doubt it’s neck tension causing that severe of a drop

 

[chkunz]

This is something huge, none of the small stuff. When you find the problem it will be something significant.

If your were shooting on an electronic target did you notice the velocity data?

 

[rogn]

Had an issue @ 600yds witha different brand primer that snuck in. Theat shot was over 7" low to a sub 1/2 minute group. Your issue is repeatable so bad batch or mixed primers, some part of rifle is unscrewing, maybe but not probable, or another strong contener; a scope that has things coming loose. Two obvious scope failures Ive had in past resulted in "2 grouping" both groups small but separated by a significant distance.
Also ake into consideration thatthe incidence is increasing in frequency. That does more to suggest a scope rolling belly up.

 

[Tim s]

Wind switched to a headwind

 

[gme]

hmm, I suspect that if your issue was caused by a huge velocity difference (whatever the reason, undercharge, different powder, oily primer, light strike etc) that would result in 2.5 feet of elevation on target your rifle would probably have short stroked, assuming it didn't? A brass or bullet defect would have manifest itself in an off call shot in any direction not just elevation, if it happened to me I'd look at the possibility of a failure in my optic.

 

[ballisticxlr]

Is it at all possible that there was shooter error involved? I know, I know. We're all the best shots in the world here right. Not one of us ever makes a mental or physical mistake. Well, nobody except me I guess and every other shooter I've ever been squadded with at a match. I have the same kind of thing happen, though usually I'll miss something like 20 inches high. I don't look at my gear, I know that somehow I screwed that shot up. I might not know how exactly I did it but I only look at me when that happens. When it's something small, then I'm looking at gun/ammo/optic consistency.

I guess you could have contaminated some powder but I'd think you'd notice the difference in the bang/kick/primer/ejection.

 

[ebb]

I have had really low shots from what Tim suggested. A head wind rolling over the berm we were shooting into. And I think like CHKunz this is not something small. 30 inches drop is big time.

 

[Dusty Stevens]

Did you keep that piece of brass seperate?

 

[carlsbad]

I'd say it's the gun, not the ammo. That is too big a difference to be anything other than a severely short load and you've already eliminated that.

Take your BCA and your gas system apart and double check it.

 

[Twdmar]

It's not the gas system and probably not the load unless somehow you had incomplete powder ignition.
It's not a headwind or mirage giving a false image either. I would agree with the optic possibly being suspect. Throw the rifle in a rest and try to replicate the problem.
I could see a doppler chronograph being helpful.

 

[Forum Boss]

Does your service rifle have an optic?

The only time I experienced an issue like yours, with shots going 2-3 FEET high or low at 600 yards, it proved to be a scope issue. Leupold ended up replacing pretty much all the mechanicals inside the optic.