The versa max has a hybrid recoil / adjustable gas cycling system - it is very similar to the benelli Argo system that's is used by the the USMC benelli repeating shotgun - gas ports in the chamber , and a recoil spring in the stock , the versamax's recoil / action spring is very light in tension similar to a inertia gun
The major difference is that in the Remington versamax and the Argo is that the Remington has several ports and they adjust the gas pressure going into the piston depending on how long the shell is , 2.75 to 3.5