This is the O-ring used in the manufacture of Shadow Systems DR920P pistols. It is located between the compensator and the barrel.
Typical O-ring service life is 5,000 rounds, but may need replacement based on use conditions or potentially damage during assembly or disassembly.
NOTE: Use only for Compensator models, do not use for threaded barrels. If requesting O-Rings for threaded barrels, can be found in the following link. https://shadowsystemscorp.com/product/threaded-barrel-o-ring/