Case forming dies are for those who wish to form brass cases from one caliber to another.  Most case forming is done to produce ammunition that is not factory available. All wildcat cartridges and many obsolete calibers fall into this category. Case forming dies are designed to change the shape of the original parent case in small increments to avoid overworking the brass and to prolong case life. After using the proper form dies and/or Form & Trim Die, full length resizing is required.