This is an interesting gun. Rick Island Arsenal Museum showed this gun today,
Its so odd that even they have no idea bout its history and have asked for help via the video the above screen shot it from.
They do know it was developed by a LTC Hamilton while he was stationed in Panama. That’s about it. They don’t even really know if this was developed for testing or some program or anything. They do mention the gun is a combination of several pre existing guns. You can see the M3 “grease gun” mag well and ejection port. They say the rear of the receiver is a home made affair.