I’m thinking an HK 416 in 6.5mm and a 1911 style .45 screw NATO interchangeability.
Didn’t the military adopt the .45 ACP about a hundred years ago to replace the .38 Special, which was found to be inadequate in the Phillipines? It looks like we’re re-inventing the wheel. (And I’d take a .38 over the 9mm any day.)