Mike, I was tempted to take a stab at that myself but it involves trig. Having failed that class, and gone on to an ignominious non-mathematical career I'll defer to Don A.

The actual muzzle speed is a good bit less than target speed, and it's the angular rate of change that matters - or in our case doesn't matter because it's so tiny.

"The price of good shotgunnery is constant practice" - Fred Kimble