Why am I on and off the target?

[hellcat370]

On the advice of my boss, who's an outfitter, I decided to put this question to y'all and hopefully find a solution to my problem.
Shooting at 663 yds yesterday with a Barrett M82 .50 cal 750 Amax and Savage 111 LRH 300 Win Mag with 200 gr ELDX. 77 degrees, roughly 50% humidity, 900' elevation, little to no wind. First rounds with the 50 hit top edge of target 14" high, holes are 5" apart. (I'm firing both rifles back to back in groups of two rounds before driving to the target to check groupings. 5-15 minutes between firing) Only 1 round from the 300 finds the board low right 8". Target is 24"x 30". Readjust the 50, one round near center, other off board. Same with the 300 on the second and third groups. Third and fourth groups with the 50 completely miss. Fourth group with the 300 results in both rounds a few inches high and just right of center, 4" apart. Brought the 50 back to 350yds after an hour or so, and it was hitting almost a foot higher than expected. Still holding groups.
This is not the first time I've shot these rifles at this range under similar conditions. Only difference was I usually use fmj's in the 50 and this time was the Hornady Amax. Everything factory loaded. I did not notice a mirage at the time, but I wasn't looking for one either. Upon further inspection behind the target, it looked like some rounds were going quite a bit higher, 2-3' difference in impact on some trees.
How am I getting vastly different POI with two different rifles in the same condition when I normally don't have these issues? I'm not a class trained long range shooter, just learned by research and doing, so any help is appreciated. Might help me get my first bull elk this fall as well!

 

[Lionel Boyd Johnson]

Once you've confirmed that the scope rings and mounts are correctly tightened, and the action screws are at the correct torque; I'd suspect inconsistent "hold" on the rifles. Everything from where and on what the forend is resting to shoulder, cheek, and hand grip pressure must be the same shot-to-shot. You mention "5 - 15 minutes between firing". It can be very hard to reproduce how you are holding the rifles with this much elapsed time between shots. Also, be sure to grip the forend of the rifle behind the front rest (hopefully, padded) with the non-trigger hand, just like you would under field conditions. Don't use that hand under the toe of the stock. Remember all contact pressure with the stock must be consistent. Let us know how you resolve the problem. -Ed

 

[hellcat370]

I took the 300 back out this morning. Out of ammo for the big gun at the moment. Similar results, though fairly consistent. I'm just shooting off my pack as I would under field conditions. I did make some adjustments to the scope, which helped some. Groups were consistent and moved when I made adjustments. How much does mirage come into play at this distance? I noticed some this morning.

 

[BallisticsGuy]

Mirage doesn't affect bullets. It's the result of something that may, it can be either air movement between your muzzle and your target or air movement between your objetive bell and your muzzle. The former can change bullet impact directly after the shot, the other make the target appear to move before the shot. Sounds to me like you need a mentor. Someone to take you aside and find all the stuff you're doing wrong and whip it out of you. Then you'll find debugging to be easier since you won't be in the equation.

 

[hellcat370]

Won't deny that! Been wanting to take some classes for a while. Might look into it when I get to Idaho next month.

 

[Lionel Boyd Johnson]

As BallisticsGuy said mirage can and often does make the target appear to be somewhere it isn't. This alone could explain some of your odd shots. When the wind is calm, the mirage may seem to "boil", with waves going up. A right or left wind will cause the waves to go down-wind. Try to avoid shooting when the mirage is boiling; and learn to use the horizontal waves to estimate the wind direction and strength. You can use a higher power spotting scope to "read" the mirage more clearly than with your rifle scope alone. While you work this out, you could put some wind flags up at intervals between your shooting position and the target. Wind close to you will move the bullet farther laterally than wind nearer the target, so have a couple of wind flags within the first 75 yards or so. Compare what the flags are doing with what the mirage is doing. -Ed

 

[243winxb]

shooting off my pack

Try a real benchrest type rest.