there is a good write up on these guns and their development in the book the worlds fighting shotguns. it is a brain child of a police officer. he worked on the idea and the original prototypes. in the late fifty's and got high standard to build them. the original gun was on a Remington receiver i believe . it was the late sixty's before a factory gun was made. the way our law is written we would be afoul of bull-pup regulations if one tried to make his own . if you have specific questions i would look up the write up and see what it says .