Well, here's the thing. There was a Springfield Armory (the actual armory) project in the 1950s (60s? Can't remember) called SALVO that used a triple barrel and duplex cartridge to push a 5.6mm round at ~5,000 FPS. And then someone in the Army conflated that round with 5.56 and sold the M-16 to the brass based on the old performance numbers.
Well, here's the thing. There was a Springfield Armory (the actual armory) project in the 1950s (60s? Can't remember) called SALVO that used a triple barrel and duplex cartridge to push a 5.6mm round at ~5,000 FPS. And then someone in the Army conflated that round with 5.56 and sold the M-16 to the brass based on the old performance numbers.