Abstract: The present invention (2) is a small arms caliber and/or power reducing device for using a primer (18) to fire a projectile (21) and thence directionally guide the trajectory of said projectile from a firearm (1) the barrel bore of which may be substantially larger than the caliber of said projectile. The present invention may be adapted by selecting appropriate external dimensions, internal dimensions and form factors to enable its use with any existing breech loading small arm chambered for center-fire cartridge ammunition. The present invention may be constructed of two parts: 1) An adapter sleeve (10), and 2) a chamber (14), and intended to be re-loadable for multiple uses. Alternatively, the present invention may be configured as a single piece, fabricated with a primer and a projectile disposed therein, and intended to be used one time and discarded. Suitable primers may include Standard Small Rifle or Small Pistol Primers, Standard Large Rifle or Large Pistol Primers, or Standard 209A Shotgun Primers.