@James K
FWIW, an story. At the time when the AR-15 was under consideration by the Army, and being hyped by Colt as the most powerful rifle in the world (yes, they said that!), I encountered a Colt rep at the NRA convention.
I asked if he knew of a source for FMJ bullets as I would like to do some testing by loading my .22-250 down to .223 (it wasn't 5.56mm yet) specs.
He went ballistic, practically screaming that I was crazy, that nothing could be loaded DOWN to .223, that it was the most powerful round ever developed, that even a miss by a foot would disable a man, that it would shoot through brick walls, that it would penetrate tanks, etc., etc. He was so funny, I just walked away, laughing. But the politicians (notably JFK) believed that nonsense, and that is how, for better or worse, the U.S. got saddled with the 5.56mm.
Jim