Re: What is acceptable velocity variation
In my experience chronographing loads, the cause of large muzzle velocity spreads has nothing to do with the ammo or rilfe. It's how inconsistantly hard the shooter holds the rifle against his shoulder. I've seen a 55 fps difference in average velocity between me and a friend shooting the same rifle and 30 caliber magnum ammo. I could easily cause a 40 fps spread myself holding that 13 pound scoped magnum hard to soft to my shoulder. A friend reported he an another person had over 90 fps average fps difference shooting the same .308 Win. ammo and rifle. Therefore, I'm usually skeptical about quoted fps spreads for a given load.
If the huge human variable's removed and the ammo's shot in a fixed barrel or a repeatable rest, muzzle velocity spreads will be much smaller. Which is why arsenal and some factory ammo's tested with fixed barrels in universal receivers.
Ballistic software will show 180-gr. bullets from a .308 Win. will have about 3.7" change in drop for each 10 fps difference in muzzle velocity at 1000 yards. 30 caliber magnums' drop difference for the same velocity change and range is about 2.3 inches. At 600 yards, the drop numbers are 0.9 and 0.6 inch, respectively.
But often that difference is compensated for by bullets leaving on the muzzle axis upswing of its whip. Slower bullets leave later at a slightly higher angle and strike the same point as their increased drop down range is corrected for.
If you ammo's shows a 20 to 30 fps spread in velocity, that's good. If it's more, that may not be a problem. Your barrel may well whip such that compensation happens in your favor. Test your ammo at long range and you'll probably be surprised at its performance. I've shot more than a few rounds from prone through a chronograph and at 1000 yards, the vertical shot stringing was less than ballistic software calculated it to be. My barrel whip was compensating for the velocity differences.