Back in the 70s, this was originally the Individual Multi-Purpose weapon or IMP. It was chambered in .221 fireball and was designed for the airforce. A PDW for pilots. It was designed to be shot from the arm, but they also did consider shooting it from a sling. Similar to SAS technique, the shooter would press out and the tension from the sling would stable the firearm.
Bushmaster took the design and chambered it in .556. It is gas piston driven. Barrel length is 11.5″ weighs 4.25 lbs. OAL 20 inches. Retail back in the 70s was $299.50 and was offered in semi as well as select-fire.