An automatic and semiautomatic small caliber conversion system for automatic or semiautomatic shotguns and rifles such as, for example, M-16 and AR-16 rifles, including a uniquely three element structure and associated ammunition feeding means to permit the use of smaller caliber, inexpensive ammunition in rifles and shotguns designed originally for larger caliber ammunition. The system includes a bolt/receiver assembly which easily replaces the bolt carrier of the, for example, M-16 rifle, and a magazine assembly which can be inserted by hand into any standard, for example, 0.223 round M-16 magazine. The bolt/receiver assembly is made up of three sub-assemblies, a receiver, a bolt, and a recoil spring. The receiver sub-assembly includes a section of a circle with a short barrel section in the front which fits into the M-16 chambers, a receiver lug at top rear to locate it within the M-16 chamber and prevent its rotary or linear motions and a guide rail. The bolt subassembly is cylindrical in shape, is guided by the adapter receiver guide rail and is encompassed by the M-16 receiver so that its only free movement is front to rear for ejecting and chambering cartridges. The recoil spring sub-assembly consists of a guide rod and spring. The spring is to return the bolt to battery after firing and also to hold the guide rod into a hole in the receiver lug. The recoil spring subassembly holds the receiver and bolt together as an assembly.