I don't know what you call it but it is probably the piece that goes ito the butt of your gun when it chambers a new round.The long piece of metal that comes off the back of your bolt goes into a small tubular piece that goes into the butt of your gun. Everytime I've had a problem with a gas shotgun chambering a round, that's what is was. Just clean it up and lube it with a tiny bit of oil and work it in and out, then remove excess oil with a rag. I do this everytimeI clean my shotgun. I'll bet a dollar you don't have another problem after that!! Good Luck!