Fireforming
Firing a case so chamber pressure expands it to the chamber's exact shape, a routine step with wildcats and Ackley Improved designs.
Fireforming is the act of firing a cartridge so that chamber pressure blows the brass out to fill the chamber completely, taking on its precise dimensions. It is the standard way to convert parent-case brass into a new shape, most famously for an Ackley Improved chamber, where firing sharpens the case shoulder and reduces body taper. The first firing forms the case; subsequent firings simply maintain that formed shape.
This step is fundamental to running many a wildcat cartridge, since the brass usually starts life as a different commercial case and only matches the new chamber after it has been fired once. Shooters sometimes fireform with mild loads built for the purpose, and the formed brass that results fits the chamber tightly, which tends to improve case life and consistency on later loadings.