cartridge conversion

A caliber conversion device is a device which can be used to non-permanently alter a firearm to allow it to fire a different cartridge than the one it was originally designed to fire. The different cartridge must be smaller in some dimensions than the original design cartridge, and since smaller cartridges are usually cheaper, the device allows less expensive fire practice.
Alternative names sometimes imply the type of dimensional difference. A chamber insert may be used for a shorter cartridge of similar base diameter. A supplemental chamber or cartridge adapter is typically used for a shorter cartridge of reduced diameter. A cartridge conversion sleeve may include a short barrel of reduced bore diameter. Shotgun conversion sleeves may be called subgauge inserts, subgauge tubes, or gauge reducers.
Sleeves intended for rifle or handgun cartridges may have rifled barrels. Additional variations may allow centerfire weapons to fire rimfire ammunition and/or retain autoloading function with the smaller cartridge.

View More On Wikipedia.org