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.

Guns.com video [5:40] https://youtu.be/MfgMQlx76hU?si=