They used Remington 870s with a metal folding stocks for guard duty in the Marines back around 1980. I had to "teach" a private how to "unload" one when he "unauthorized " racked one into the chamber BEFORE he challenged me and a couple others. We were where we were suppose to be and he was not. They issued him the shotgun , showed him how to rack one into chamber (carried it condition 3) but no other training.