Abstract: A conversion kit for allowing an M16 rifle to fire .22 caliber ball and bk ammunition in semi-automatic and automatic firing modes, comprises a bolt replacement which replaces the standard bolt in the rifle. The bolt replacement moves in the rifle receiver and is engageable with a sear trip bar of the conversion kit. The sear trip bar interacts with an automatic sear of the M16 rifle to permit automatic fire. A spring loaded detent is provided in the conversion for engaging a detent slot in the replacement bolt, to hold the bolt face against a chamber insert, to avoid bouncing of the bolt for automatic and burst firing modes. For the firing of blank cartridges, a shaft is provided in a chamber insert for reducing the diameter of the chamber insert to preclude the feeding of ball cartridges. An orifice in the shaft allows for escaping gases to leave the blank cartridge, but produces back pressure so that the replacement bolt can be cycled for blow back operation.