A shotgun ammunition conversion system has a detachable magazine well having a sleeve defining a rectangular passage adapted to removably receive an ammunition magazine, a boss extending forward of the sleeve and at a level above at least a portion of the sleeve, the boss being adapted to be received in the rear aperture of the magazine tube, and a tang extending rearward of the sleeve and defining a tang aperture operable to receive a fastener associated with a shotgun frame to secure the magazine well to the shotgun with the sleeve proximate and aligned with the loading port when the boss is received in the rear aperture of the magazine tube. The boss may have a lower cylindrical surface portion operable to contact a lower portion of the magazine tube adjacent to the rear aperture. The boss may be a cylindrical body.