Rule of thumb
A rough rule of thumb is 35-50 fps per inch for handguns, and 50-75fps per inch for rifles, BUT it is only an approximate. Each gun is a law unto itself.
I have seen a 100fps difference between three different guns with the same barrel length firing the same ammo. This is a little outside of the average, but it does happen. So it is possible for a "fast" carbine to equal the velocity from a "slow" rifle length barrel. The only sure way to tell for certain is to chronograph your gun/ammo. Anything else is just an estimate.
All else being equal (and it almost never is) bigger bullets tend to work better.