The Benelli semi-autos have what they call an 'inertial' action which means a more sophisticated sort of a blow-back mechanism which actually locks, but it cannot shoot the lighter kinds of skeet shells reliably. My own preference is the Beretta Xtrema-II which involves a very sophisticated gas mechanism with a heavy spring, a bypass valve, and a piston RING if you can believe that; they claim it's self cleaning and it isn't much of a lie: cleaning the piston and the chamber it moves in is a ten second deal and you still have to clean the barrel. It's ungodly strong and can shoot 3.5" shells if you want to.
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