BudsGunShop.com - gas piston

John, the light piston is designed for lighter loads such as target loads, low-recoil loads, and most other common loads while the heavy piston is for more powerful loads such as most 3 inch shells, slugs, magnum turkey loads, etc. (Technically the light piston is intended for load pressures up to 750 bar if that information is available to you) It may require some experimentation with different loadings to see which piston gives the most reliable operation since there are so many different loads available. In reference to James G's answer, there is some confusion as to the operating system of this gun; it is listed as inertial driven but does have a piston part that is nested over the magazine tube where the barrel band sits over it.

there are no gas pistons in this shotgun it is a inertia gun

John, this 12GA shotgun operates with an inertia style action and not "Gas Piston". Inertia operation involves a bolt carrier and inertia spring that are not rigidly fixed to the rest of the gun. When the gun is fired, the recoil pushes the gun backwards against the inertia spring and bolt carrier that are effectively floating in space inside the receiver. This rapid recoil impulse hits hard enough and fast enough to compress the inertia spring about 4mm, but because the spring is so incredibly stiff it’s still able to store enough energy to send the bolt flying backwards where it will compress the return spring while extracting the spent shell and re-cocking the hammer. ****Because inertia guns don’t need to hide any gas pistons or external linkages they tend to be thinner and less bulky than gas guns. ****