The term fire forming in firearms refers to the process of reshaping a metallic cartridge case to fit a new chamber by firing it within that chamber. This might expand a cartridge to a new size, such as a wildcat cartridge

Wildcat cartridge

A wildcat cartridge, or wildcat, is a custom cartridge for which ammunition and firearms are not mass produced. These cartridges are often created in order to optimize a certain performance characteristic of an existing commercial cartridge.Developing and using wildcat cartridges does not...

, or just to the chamber of a specific gun.

Fire forming a wildcat differs from the normal manufacturing process; in that it relies on firing a loaded cartridge

Cartridge (firearms)

A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head or at its rim . Electrically...

 of differing dimensions than the chamber which it is being fired in. After fire forming, the spent case will take on the new dimensions of the firearm's chamber. Fire forming is the final process in creating a wildcat or an improved cartridge.

There are two methods of fire forming. One method is to cold form a parent case using forming dies, creating some form of headspace

Headspace

In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge to the face of the bolt. Used as a verb, headspace refers to the interference created between this part of the chamber and the feature of the cartridge that achieves the correct...

, load the case and fire the cold formed cartridge in the chamber of the firearm. This first method is the most common and will create a wildcat cartridge. The second method is to fire form a factory cartridge by using its factory headspace to headspace on. The factory cartridge is then fired in the chamber of the firearm. This second method will create an improved cartridge.