you cut of the base of the brass and fill the primer hole with soft solder? when making 12 mm pinfire cases I was using 6,8/11 rimfire tool blanks in cut down 308 win brass. the fired 6,8 mm case as a sleve was inserted into a hole in the 308 win making the inner space for the cap. it reduced powder capacity but I could load the cartridge on the rock chucker like any other ammo.