"The M4 Super 90 is an Italian semi-automatic shotgun manufactured  by Benelli Armi S.P.A. The M4 was the first gas-operated shotgun produced by Benelli. Its  function is designed around an entirely new method called the ""auto  regulating gas operated"" (ARGO) system. The ARGO system on the M4 opened  the door for Benelli's development of the R1 rifle line. The design  uses two stainless-steel self-cleaning pistons  located just ahead of the chamber to function opposite the rotating bolt, thereby eliminating the need for the complex  mechanisms found on other gas-actuated automatics. Benelli accomplishes  this level of reliability through the simplicity of the mechanism. The  ARGO is a short-stroke system that incorporates only four parts. It  consists of two symmetrical shrouds containing two small steel gas  pistons. It is also self-regulating for cartridges of varying length and power  levels. It can fire 2.75 and 3-inch (76 mm) shells of differing power-levels without any operator  adjustments and in any combination. Low-power rounds, such as  less-lethal rubber pellets, must be cycled manually. The sights are military-style ghost ring and are adjustable in the field using  only a cartridge rim. The  MIL-STD-1913 Picatinny sight rail on top allows use of both conventional  and night-vision sights, while retaining use of the original sights."
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