Clay Gun

Good Luck with it  FYI get some spare "O" rings.

 

 

That guy's "O" ring looks worn because he installs it incorrectly! The O-ring is to sit in the recessed groove on the magazine tube. You don't roll it down the magazine tube. It doesn't ride up and down the tube in front of the piston and piston seal.  It's designed to serve as a gas seal. When the barrel is installed the O-ring sits in the gas cylinder and serves as a seal to prevent gas from escaping forward. The vents in the barrel direct gas rearward directly to the piston and piston seal.

PS - If you lose/break an O-ring, Teflon tape can be wrapped in that groove a few times and in a pinch and it will get you through a day of shooting.