Purpose built for close quarters combat. Good article but I question where it says it can still penetrate heavy body armor. You lose a lot of muzzle velocity by the time you get down to a 5.5” barrel, between 50 and 100 ft/sec for each inch on a 5.56. Plus these would be subsonic rounds by design, so more like a heavy pistol round. But I don’t have the data on blackout to say for sure. Otherwise, cool gun porn.
I will stick to energy…mass & velocity.