A conversion kit for converting a semi-automatic or self-loading handgun of a certain caliber, such as a Colt .45, is modified to allow the use therein of cartridges of a different, usually smaller caliber, thus to allow less expensive training on the handgun, wherein only the barrel and magazine need to be replaced, the barrel having thereon a sliding extractor that cooperates with the original slide in the ejection and reloading of cartridges, and an off-axis bore adjusted to compensate for the difference in the reloading mechanism and the lesser rise of the handgun when firing the lower caliber ammunition, thereby allowing retention of the sighting, safety features, and mechanical functions of the original handgun while closely duplicating the original weight.