Abstract: A magazine adapter includes a base that is structured to fit within the magazine well of a firearm, and to accept a magazine designed for smaller caliber ammunition than the original firearm. The adapter includes a bolt catch actuator slidably mounted therein. Magazines for which the adapter is intended include an upwardly extending rear projection of the follower, projecting from an opening in the rear of the magazine housing when the follower is in an upper portion of the magazine. When the upwardly extending projection pushes upward on the bolt catch actuator, the bolt catch is actuated, holding the firearms bolt in a rearward position. The magazine adapter thereby facilitates use of the original firearm controls with a caliber conversion unit, or with an upper receiver and barrel combination of a different caliber.