I was going to the gunclub a couple years ago ... and a guy had brought a replica Gatling Gun, which he had built. He let everybody at the club that day have a turn firing it.
I know, not a true "minigun" ... but it was still fun.
He built the barrels and everything from scratch. He had been blacksmithing for over 50 years. He had quite a collection of working firearms he had personally forged. Besides working replicas based off US Patents, he had designed and forged some custom revolvers and rifles himself.