Wildcat Cartridge
Also: Wildcat
A nonstandardized cartridge created by reforming or modifying an existing parent case rather than one made to an industry specification.
A wildcat cartridge is a round that has not been standardized by an industry body such as SAAMI, created instead by handloaders and gunsmiths who reshape a parent case to new dimensions. Common moves include necking the case up or down to a different bullet diameter, blowing out the body for more capacity, or changing the shoulder angle. Because no factory loads it, a wildcat lives entirely on reloading, and the shooter forms brass and works up data from scratch.
Wildcatting is how a great many mainstream cartridges began before they were eventually adopted as factory rounds. The appeal is tailoring ballistics to a specific goal, whether that is higher velocity, better case efficiency, or a bullet diameter no standard case offers. The cost is the labor of fire-forming brass and the lack of off-the-shelf ammunition or load manuals.