Date: Dec 18 2005
Fireforming" is the process of firing a metallic cartridge in a chamber slightly larger than the cartridge. Upon firing, the brass case expands to fit the chamber walls. Normally this is done to "improve" the original cartridge, giving it slightly more powder capacity and hence velocity potential. But sometimes fireforming is done to create useable brass for an obsolete cartridge.
Before obtaining a rifle that requires fireforming cases, I'd advise reading my previous column about "Ackley Improved" cartridges. I've experimented with several of these, and while velocity can be increased somewhat, it's rarely more than 100 fps. Usually, a factory cartridge of the same bore diameter can be found that will do better, if extra velocity is really needed or desired.
In general, I've come to regard fireforming as a waste of time and powder – unless used to get an older rifle up and running. I have an old Lee-Enfield .303 rifle that belonged to my favorite uncle. The chamber is so generous that even the brass of factory ammunition tends to separate after one firing. The only way to handload for this rifle is to fireform cases to fit the oversize chamber and only neck size them afterward.
Notice that "bullets" aren't listed in the above sentence. You can fireform cartridges without using up bullets. The big trick is to make sure the "new" case is fully fireformed. Otherwise, excessive headspace can be created between case and chamber, at the least guaranteeing short case-life, at most creating a dangerous condition.
The "bullet-less" method I've used to form rifle cases is adapted from The Handloader’s Manual of Cartridge Conversions, by John Donnelly. The primed case is charged with 10 grains of Alliant Bullseye, a very fast-burning powder. Cornmeal is poured into the case to the mouth, then packed tightly in the case mouth. A little Elmer's Glue is applied to the top of the cornmeal and allowed to dry.
Before firing apply a thin layer of light gun oil to the case. This helps it fill the entire chamber, preventing the headspace problems noted above. This method is particularly useful when forming cases in "improved" chambers that have been reamed out without setting the barrel back one thread. While factory ammunition, or handloads with a bullet instead of cornmeal, can be used in such chambers, the brass tends to either turn out short or to stretch too much, even if the case is oiled.