1st up, a sidearm weapon firing a (sort of) rifle round.....
My version of the Bushmaster doesn't look anything like what yours does.
This weapon was considered as a sidearm for heavy weapons crewman (machine gunners & mortar gunners) and air crew back in the late 70's. I used it at the (then) Arctic Test Center at Ft Greely, Alaska, 'Home of the Rugged Professionals.'
Uses standard M16 magazines which can rotate 45 degrees pitch. Oh, it's very LOUD.