The idea is to create a shooting "window" that has two photo transistors that are a KNOWN distance apart. When the bullet passes the first "eye", which blocks some of the ambient light for a fraction of a second, this type of situation can be detected. A start and stop sensor of known distance...hmm, sounds like you know a velocity! That is the EASY part, the hard part is distinguishing muzzle blast gas from the true bullet. There are a bunch of these out there already, but I won't mention the company that I work for. Also, most contain advanced ballistic calculators that can calculate drop tables and such.

Anywho, hope that gets you in the right direction.

Mike