BENELLI M4 ENTRY LE 12GA 14″ SEMI-AUTO SHORT BARREL SHOTGUN. On Form 3. This is the same Benelli M4 used by the U.S. Marine Corps and it can now be your law enforcement 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. The metalwork is chrome-plated, anodized or coated with heavy phosphate to prevent corrosion under the most grueling conditions