Guns: Gas ring etiology
Perhaps no part of Eugene Stoner's AR-15/M16 design has been the subject of as much mystery and misinformation as the gas rings:
They look benign - three thin metal rings set into the end of the AR's bolt - but people have ascribed all sorts of malfunctions to them. Some people say you need to stagger the gaps in the gas rings or you'll have decreased reliability (which is wrong, based on my experience). Some say that carbines can run even with a gas ring or two missing (which I've never tried). I'm not sure what the real answer is, so I'll quote Bartholomew Roberts: "If the tiny gap in the gas rings aligning makes the difference between your rifle functioning and not functioning, you've got a problem somewhere else."
That being said, there is a test for seeing if your rings are worn out, and I do replace rings from time to time, just to be safe. With the bolt carrier group fully assembled, extend the bolt and stand the entire BCG on a flat surface:
If the bolt closes in under the weight, the rings are worn (as in the picture below):
Time for some new gas rings. Unless you have a very well-stocked gun store that specializes in black rifles, you'll probably need to order gas rings online:
Pop out the worn gas rings with a pair of pliers. Gently work the new ones into the bolt recess, taking care not to bend them too much. It helps to get one end of the ring into the recess and rotate the ring in, like a keyring.
After the deed is done, your BCG should stay extended when stood up on a table, and any gas ring demons should be thoroughly exorcised: