The AR-15 handguard cap sits at the front end of a drop-in handguard and secures it to the barrel or front sight base. It holds the front of the handguard in place and keeps it from shifting during use. Builders install it on standard two-piece drop-in handguards on mil-spec AR-15 upper receivers. It fits over the delta ring barrel nut or front sight base and works with carbine and rifle-length polymer handguards. Shooters replace it when upgrading drop-in handguards or restoring a factory configuration.