NORCAL-NOODLER

I just purchased a extrema 2 and saw the Beretta forum was not as popular as this one. So I posted here!! Anyways after a misfire I took the gun apart and found a mystery O-RING !!! It was located (I really don't know the names of each part yet) on the outside of the magazine tube between the spring and the piston??? This happened after two boxes at the trap range, one disassembly, and then two missed pheasants. After scouring through the manual and not seeing a word or picture on the o-ring we tried firing the gun without the o-ring and BANG off it went. But seeing as I can't afford another gun just yet I figured I find out about this mystery part before firing it again!!!! Thanks for any help you can give or where to go for info !!!