Case forming dies are for the reloader interested in forming cases from one caliber to another. This is done either because the case needed is prohibitively expensive or it is otherwise unavailable. The specific steps necessary to form cases will vary greatly depending on what changes must be made to the parent case and may include initial forming, separate base forming, annealing the brass, making the neck larger or smaller, cutting the cartridge rim and fireforming the case in the firearm's chamber to obtain the finished caliber.
Case forming dies are designed to reduce the case by steps to eliminate case stress or collapse and to prolong case life. There will be one or more form dies for each caliber. After forming and trimming, cases must then be full length sized, using the full length sizer in the reloading die set for the finished caliber. RCBS does not recommend using full length sizer dies for the actual case forming process. Case forming dies (threaded standard 7/8 x 14 unless otherwise noted) will withstand the normal case forming pressures encountered but are not intended to swage the base of the case. When that is required, a special Base Form die is needed. Base Form dies are unthreaded and require the use of a 3-ton arbor press.
- Some cases will need to be shortened after initial forming. A Trim Die is designed so that a mill type file can be used to trim the case to the correct overall length without damaging the surface of the die.
- Frequently, changes to a case will result in brass that is too thick at the case's neck. A Neck Reamer, if needed, may be chucked in a tap wrench or in a hand drill. A drill press is not recommended.