The Benelli M4 is a gas operated semi-automatic shotgun. It’s currently one of the shotguns in use by the United States military, and utilizes a short stroke design with two self cleaning pistons to function on the opposite side of the rotating bolt. This is a much simpler system than many other semi-automatic shotguns, which should theoretically enhance reliability. Being a semi-automatic, the M4 will have a much faster rate of fire in contrast to other semi-automatic shotguns. That being said, the M4 is also a very expensive weapon, so if you’re on a budget a pump action would probably be better cost wise.