I just ran the numbers on this. IF both guns weighed 7.5 pounds, and you fired a 1-oz load from the 20 ga. and a 130-grain bullet from the .270, both would give about the same recoil- between 17.5 and 18.5 foot pounds... BUT the length of time over which the energy is applied will be different, due to the difference in MV of the projectile mass and the jet effect of the gases at the muzzle..... (the .270 will work faster, and the shotgun will "push".....)