The 12/14 that Im aware of was a brass case with 12ga diameter base with the forward 2/3 of the case being 14ga diameter. (Think bottlenecked rifle cartridge) The idea was if a bad guy stole the Police gun there was no commercially available ammo that would fit the gun. The 12/14 was the answer to the previous idea of using a regular 14ga case. The bad guys figured out you can shoot a readily available 16ga shell in it just fine.