Okay, example, my M4 (Matilda) shoots .20's at 407, and .25's at 386.
.20 / 407 = 1.54j
.25 / 386 = 1.73j
Now of course there is a difference between muzzle velocity and at target velocity. Getting hit a 10 feet versus getting hit at 50 feet will of course have less impact as the BB loses velocity.