I've ran across these old Marlin pump shotguns at gun shows and gunshops and invariably,  the one trying to sale it will tell you "they use these in cowboy action shooting.  It's came up here on the Wire why are these guns not allowed in cowboy action shooting.   This video explains the safety issues with this gun and why they are not safe to shoot.     Also, he explains the forcing cone issues with firing modern ammo in any old shotgun.