The M4 is the first Benelli semiautomatic shotgun to feature Benelli's innovative and patented piston-driven Auto-Regulating Gas-Operated system (A.R.G.O.) Fitted with a blade type front sight protected by side wings and an adjustable dioptre, the barrel is internally chromeplated and externally phosphated for wear and erosion resistance . The M4 is still one of the best shotguns available today in terms of performance, choice of materials and reliability.