Here’s an old article from Small Arms Review detailing a 9mm hybrid submachine gun made from a STEN parts kit combined with a registered M16 lower receiver. It fires at around 800 to 900rpm and has a 13 ounce bolt which was turned down from an original casted bronze example. It reportedly ran flawlessly from the get-go.
Below: “On top is a standard 11.5 inch Colt Commando upper. Second from top is the FrankenSten. Third from top is a custom 6.5 inch 5.56mm upper with a red-dot sight.”
Below: Top: Unmodified STEN bolt, Middle: FrankenSten bolt, Bottom: unmodified M16 bolt
Below: Upper receiver consisting of 1″ steel tubing with a smaller diameter cocking bar housing welded on top.