Abstract: The present invention teaches an insert locking mechanism incorporated into a sub caliber action device, installable within an upper receiver of an AR-15 type firearm, and which interfaces with an upper receiver mounted forward assist for use with such as .22 caliber ammunition and in order to eliminate movement or play of the chamber insert adaptor associated with the substituted sub caliber action. The mechanism includes a generally cylindrical shaped body supported upon the action and exhibiting a plurality of radially projecting and circumferentially extending lugs which are configured to mate and seat within a plurality of grooves circumferentially arrayed within a barrel defined in the upper receiver.

Abstract: A retrofit chamber adaptor associated with a substitute action assembly incorporated into a firearm for permitting the substitution of an original designated cartridge for a substitute cartridge. An elongated body is incorporated into a barrel accessible location of the firearm and supporting a bolt in reciprocating fashion. The chamber adaptor is mounted to forward end of the body in communication with the reciprocating bolt and exhibits a modified diameter aperture extending therethrough and corresponding to an incremental modification in diameter associated with the substitute cartridge. Additional features include a receiver plate incorporated into the body and exhibiting a generally lengthwise extending notched or keyed interior established between generally parallel extending sides with identified and interiorly notched locations for seating in reciprocating fashion therebetween the reciprocating bolt.