As Shotgunjones so excellently explained the velocity is not linear but accelerating all the way down the bbl. In fact some significant amount of that initial pressure rise will occur before the shot moves at all just in overcoming its inertia & friction to send it on its journey down the tube. So some of those little bits of milliseconds are consumed in an essentially stationary position.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra