Momentum long from: Gun velocity = (bullet mass x bullet velocity) + (powder charge mass x powder charge velocity) / Gun mass x 7000. Then, plug the now known Gun velocity into the translational kinetic energy formula: Recoil energy = Gun mass x Gun velocity2 / 2 x dimensional constant.

Example:
Firearm: Mauser 98 chambered in 7 x 57 mm Mauser weighing 10 pounds.

Projectile: spitzer type bullet weighing 140 grains with a muzzle velocity of 2700 feet per second.

Powder charge: single base nitrocellulose weighing 42.5 grains with a powder charge velocity of 5200 feet per second.

8.557 ft/s = (140 x 2700) + (42.5 x 5200) / 10 x 7000.

Then, plug the now known Gun velocity of 8.557 feet per second into the translational kinetic energy formula:

11.38 ft-lbf = 10 x 8.5572/ 2 x 32.163.

Momentum short form: Recoil energy = {[ (bullet mass x bullet velocity) + (powder charge mass x powder charge velocity) / 7000]2} / Gun mass x 2 x dimensional constant.

Example, same as above:

11.38 ft-lbf = {[ (140 x 2700) + (42.5 x 5200) / 7000]2} / 10 x 2 x 32.163.

Powder Charge Velocities

Typically the charge velocity are:
Black powder: 685.8 m/s (2250 ft/s)
Pistol and Shotgun powder: 1707 m/s (5600 ft/s)
Rifle powder: 1585 m/s (5200 ft/s)
BMG powder: 1433 m/s (4700 ft/s)

However these charge velocities are approximations.

http://guns.wikia.com/wiki/Recoil_energy