A wildcat cartridge is a cartridge that uses the case of a commercially sold cartridge fitted with a bullet of a different caliber. A wildcat cartridge is usually not sold commercially and no complete weapons are made for it. However, barrels for the caliber are often manufactured by both weapons manufacturers and gunsmiths specialized in making barrels.
For example, the .338-06 is a wildcat cartridge, that has now become a proprietary cartridge, i.e. the ammunition is now manufactured commercially. It is based on the .30-06 rifle cartridge and is modified to accept .338 caliber bullets.
