The piece on the outside of the bolt body ,ahead of the bolt handle,open on the bottom & has a small ball bearing that runs in a groove on the outside of the bolt.It has a cam/ramp that engages w/bolt handle. I haven't tried to shoot it w/o it. The bolt seems to function ok.The only thing that I can see is that it may help stabilize the bolt while in battery. Any suggestions where I can get one? It happened late today Shooters & Brownells don't have it. How about a list of other possibilities ?