'Boomer,
Handguard cap, as already noted. I'll just add that there is round and triangular handguard caps. Typically, round is used on the carbine length gas system, and triangular on the mid and rifle. But there are a few exceptions.
If you use an aftermarket handguard like that UTG you posted up, or others that are used in conjunction with the handguard cap, make sure it fits your handguard cap shape. Some fit either cap, some don't.
