I had a Benelli M1 Super 90. It was a great shotgun with one exception. It was the worst kicking shotgun I ever fired. It was actually worse than a light single barrel.
I think they have done all kinds of things to try and lessen the recoil.
The M4 is gas operated and typically those have very light recoil.
I can’t imagine it taking someone’s head off tho.
A 12g shotgun will NOT blow a human head off. It’ll knock a sizable chunk out of one, there’s simply not enough moving mass and energy. Even a neck shot wont sever a human head. Silliness.