Now the same Benelli M4 used by the U.S. Marine Corps can be your home defense shotgun of choice. When the U.S. Marine Corps went shopping for a shotgun, they bought the M4 – the 12-gauge shotgun that functions the first time, every time. Its unique Auto Regulating Gas Operated (A.R.G.O.) system developed by Benelli engineers has dual stainless steel, self-cleaning pistons located just ahead of the chamber that operate directly against the bolt assembly.