My lack of info clustered this a bit, my bad. I group @ 2" or better, bench rest and groups don't jump around at all. It's the point of impact vs point of aim that changes each trip which got me thinking that zero was going off - groups would still be fine, POI would would be slightly off from expected. I was questioning ammo, environment, etc. The aimpoint is 2moa is on a larue mount, no movement.
You can see a shift in point of impact from point of aim. It really depends on the differences in the ammo and if your gun likes that ammo. It also is dependent on the quality of the ammo.
If your buying some discount bulk pack ammo you could see velocity difference vary widely in one box. I lost all my data when computer crashed. I did some velocity testing on many different brands.
The worst was a 1k round case of loose 5.56(m193) it ran as hot as 3100fps and as low as 2700fps.
Now the issue of knowing how to shoot , is it you , the gun , ammo. This takes some time.
The best thing to do is have some one who really knows how to shoot shoot your rifle.
My dad taught me how to shoot. He said it's how he was taught to shoot in the army.
1. If the rifle was on target at 25 yards that was good enough. I will explain.
My dad would place a large sheet of paper at 25 yards. With a 2" bullseye. I would shoot from prone position aiming center of the pull. I would shoot 5 3 shot groups.
If your aiming the same every time the groups will be in the same spot. My dad refused to adjust the sights until those 5 groups where with in a 3" group any where on the target.
Only then would he adjust the sights to point of aim point of impact.
I found a link out there on the inter web about the army doing the same thing.. basically you would not "ZERO" your weapon until you could group consistently. It seems you can group
By memory I belief you had to shoot 5 consecutive 3 shot groups with each group be less than 1.77" before moving on to zeroing.
Once you do this as mentioned buy some ammo find a type/manufacture and if possible same lot number and buy as,much as you can.
Even shooting a different manufacture the jacket could be different thickness,metal and create problems.
I do believe if your getting 6" differences from center to center of different ammo groups at 50 yards something might be amiss.
I started a thread touching on this a bit. It's not so about impact shift but just what you can see with different ammo in different guns....with a bit of reloading tossed in.
http://www.northeastshooters.com/vb...-A-quick-comparison-of-different-bullets-ammo
Basically zero for the basic ammo you will use the most. M193 or what have you.
I shoot for groups when I'm just range plinking. I don't have time to fiddle with adjusting.
My last note. Try to keep records on what ammo does what.