Question: Why do I always hear that heavier guns recoil less?
Answer: Because it is true! First of all it’s important to differentiate between “measured” recoil and “felt” recoil. If you had two shotguns of the same weight, one was an automatic and one was an over/under, and you shot the same shell through both guns, the measured recoil would be the same for both guns. The measured recoil is the same for both guns because the shell payload (the weight of the shot) and the velocity of the shell (the powder) would be the same. Therefore, the backward pressure generated by the shell and exerted on the gun will be the same. (Newton’s Third Law says that for every action, there is an equal and opposite reaction.)
There are only three ways one can decrease measured recoil: 1) decrease the payload of the shell (the amount of lead in the shell), 2) decrease the velocity of the shell (the powder charge), or 3) increase the weight of the gun. With increased weight, a gun has greater inertia and requires more foot-pounds of energy to move it out of its state of non-motion. Even though measured recoil was identical for our two guns (above), the felt recoil of the automatic is less than that of the over/under because the mechanism of the automatic essentially spreads out the impact of the recoil. So, when we say that an automatic has less recoil than an over-under, we are referring to felt recoil, not measured recoil.
If you want to reduce the measured recoil of the over/under without changing shells, you would need to add weight or mass to the gun. If you added a recoil device, like an ISIS system, to the stock of the over/under, it would not only reduce felt recoil by spreading out the impact of the backward pressure, it would also decrease measured recoil because of the additional weight of the installed recoil system.