All guns are different, and as you demonstrated, two identical cartridges shot through two different barrels will turn in different numbers. That's due to minute differences in manufacture. Even with barrels that are rifled consecutively, the one rifled later will have different wear and tool marks on the cutter, and it will be different from the barrel manufactured earlier.
Of additional interest is the standard deviation that we find between cartridges manufactured in the same batch. Many of us use Standard Deviation as a measure of accuracy, as it is a measure of consistency. If I find a batch of ammo with a standard deviation (Sd) of five feet per second, I consider that might be an extremely accurate batch of ammo. Normally I consider a Sd of 20 fps as the defining point. Anything under 20 is generally very accurate, anything over 20 is less accurate.
The .308/.30-06 debate has been going on forever. I consider the two cartridges practically identical at all bullet weights under 180 grains. The .30-06 generally gets the edge with heavy bullets and the .308 is generally considered more accurate. It's a toss-up. Pick one or both, it'll serve you fine.
