A conversion device for converting a firearm designed to fire a cartridge of a certain size and caliber to another size and caliber cartridge is disclosed. In some embodiments, the conversion device is suitable for converting an AR-15 type firearm to be able to fire .22 caliber cartridges. Disclosed embodiments of the conversion device may comprise an appropriately sized chamber insert to fit in the chamber of the firearm, a bolt, an extractor, extractor plunger, extractor spring, firing pins, firing pin springs, a bolt spring rod and block, a bolt return spring, slide rails, lock rings, and lock pins.