If it is a .22 cal the oring is a #009. Pretty simple fix that will probably never fail if you use the good stuff. 90 Durometer urethane is the stuff that will solve the problem permanently.
That oring is supposed to be sandwiched between the brass piece and the back of the barrel which the brass piece presses over.
Sometimes the brass piece can move off the back of the barrel in which case the barrel needs to be removed and the oring replaced and then the brass piece needs to be loctited in place.
Usually something we can do while you wait depending on how busy we are.
My Shop is in Gardena. Open hours are 11-6 M, TU, F & Sat. 310-327-3581
If you replace it with a like hardness oring you'll need to get a few as they only last so long. A hard urethane oring will last for a very long time. We've yet to see any fail.
"NO GUNS WOULD BE A RIOT"