Don’t know about a fifty round drum but the Pancor Jackhammer
(which had a brief appearance in Total Recall—the tunnel chase with the taxis on Mars) has a ten round drum and is full auto—unfortunately only a few prototypes were built iirc
You’re right. It was the Atchison, and it’s a 20 rd drum.
Carrying around fifty rounds of 12 gauge would be a stone cold b*tch.