Re: FPS or velocity from gun to gun
They should be close - BUT - throat, twist, other minor differences can add up to make your velocities different. I am also a firm believer in gun gremlins, that make what appear to be the same (but different) equipment do unexpected things.
As far as determining velocity, I saw something interesting in a post on chronographs that I intend to try. Basically, the poster (my apologies I forget who to give credit too) will zero at 100, and then shoot groups a few farther distances. By measuring the drop at these ranges, he can use ballistic programs to back-out the true velocity. Pretty smart IMO.
Hope that helps!