Roughly:
(mass of bullet*speed of bullet)/mass of gun. For a Single/Bolt/lever/pump
Higher number = greater recoil.
Different actions can change the 'effective recoil'. For example, Bolts, lever actions, and such where just about the only part moving when you fire the gun is the firing pin have the most effective recoil for the weight of the gun. Semi-autos have the bolt going back, so you'd have to figure that it.
Hmm...
(BulletM*BulletV-BoltM*BoltV)/(GunM-BoltM) Might work. Of course, it might be tough to figure out how fast the bolt's moving
Then you have recoil suppressors, But they generally work on the same basic principle as the semi-auto does, throwing a seperate mass backwards faster than the gun's moving. So the same principle should work. Of course, you still have the recoil, but it's over a longer impulse, therefore easier to deal with.
Finally Muzzle Breaks. That's beyond me for calculating.