Little care with the solid breech vs gas gun. Note that the gas port is quite a ways down the barrel and, that until the wad clears the gas port, the gas gun is a solid breech. I will agree that gas guns have different "feel" to their recoil, so do long recoil guns. They have the same total recoil per shot, though, MV and ejecta weight being equal. Is a gas gun shooter less tired after firing X hundred rounds than is the O/U shooter?

Remember that in all cases the MV and ejecta weight must be identical or you mix differing total recoil with felt recoil.