There were only three functional Pancor MK3 Jackhammer select-fire 12 gauge bullpup shotguns ever made two of which were destroyed making this particular gun the only surviving one. This gun was developed to become a new military shotgun.
It was actually tested by the US military but never adopted. This specimen is probably one of the first prototypes because it has some design elements which were improved in other Jackhammers that were submitted to the military.
Particularly, this sample has a rather difficult magazine/cylinder replacement procedure requiring partial disassembly of the weapon and it is also very heavy weighing in at 17 lbs due to the use of cast parts. Interestingly, the forearm you see in the pictures is borrowed from an MP5SD and it’s not what the Jackhammer was originally designed to use.