Based on the .30M1 case and resembles a 5.7mm MMJ, but with a more gentle shoulder. According to Dan Watters on THE GUN ZONE, the 223 Scorpion was the brainchild of Californian gunsmith Ken Bucklin way back in 1945. The above specimen is a recent remake of the cartridge, but there was an article in GUN WORLD Magazine, April 1964 that had a writeup about M1 conversions. Article below.