Rimfire, a widely used term in the field of firearms, refers to the type of ammunition used in certain guns. However, if you're seeking alternatives to express the same concept, there are a few synonyms to consider. One option is "centrefire", which describes ammunition where the firing pin strikes the center of the base. Another synonym is "percussion cap", which refers to a round with a small explosive cap on the base. Lastly, "pinfire" can be used as an alternate term, depicting ammunition that is ignited by a pin hitting the cartridge's base. These synonyms signify various mechanisms for firearm ignition and can be used interchangeably with "rimfire" in appropriate contexts.