Simple physics, longer barrels have higher muzzle velocities. Higher velocities have flatter trajectories. Flatter trajectories will yield better accuracy over longer distances. I have riles in 308 in 20", 24" and 26", the 20" needs 10.7 mils at 1,000 yards, the 26" runs 9.8 mils with the same load. The 26" barrel has the most rounds through it, and still produces the tightest groups at distance. At 250 yards, you can't tell the difference, at 1,000+, it is all the difference. Sure, barrel conditions will also effect speed, but just take a look at any professionally mapped barrel vs length graph. Best speed is achieved from longer barrels. I shoot multiple calibers and with several, I have multiple barrel lengths, I find this true with all of them. It does not mean they won't shoot great at distance, because they all do. But the ones with longer barrels produces the best accuracy.