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 100 may comprise an appropriately sized chamber insert 1 to fit in the chamber of the firearm, a bolt 2, an extractor 4, extractor plunger 3, extractor spring 5, firing pins 6, firing pin springs 9, a bolt spring rod 10 and block 11, a bolt return spring 12, slide rails 14, lock rings 17, and lock pins 15.