A rifle barrel adapter that ensures adequate mounting overlap for muzzle mounted grenades for proper firing alignment is described, which adapter is formed of only four parts: (1) an elongated unitary body with a narrowed cylindrical front section upon which the muzzle mounted grenade is mounted, which elongated unitary body has an enlarged rear section that is mounted about the muzzle end of the rifle barrel over the flash suppressor; (2) a unitary locking mechanism to secure the elongated unitary body to the muzzle end of the rifle; (3) a bias spring to maintain the unitary locking mechanism in place, and (4) a locking pin to hold the unitary locking mechanism in place within the unitary body.