Several years ago a buddy had a pair of .280 Ackley Improved Rifles. One was a custom Remington 721 with a 26" Douglas Premium Barrel. The second (chamber cut with the same reamer if I remember right) was a Thompson / Center TCR 87 with a 24" Barrel. In those two barrels no matter what load we chronographed in both rifles the 24" T/C Barrel produced more velocity, about 100 FPS more.
With that said you have to throw the individual rifling into the mix also.
Larry
