I have one using a wildcat round based on the .357 Maximum case, made by running in a .223 Ackley reamer to the depth of a .221 Remington Fireball. It ends up essentially as a rimmed .221 Rem Fireball Improved. I used the original good condition rimfire barrel and shoot .223 bullets instead of .224. Had to change the firing pin to center fire and bush the block. Nice cartridge.