What does that 6mm case look like? Long and thin, or short and fat? I know "short and fat is where it's at" in bolt hunting and sniping rifles, but in an autoloader, case geometry is optimized by many other factors.
I know Armelite makes a .243 AR-10, that would be fun to shoot, but not if it's an orphan. I'd love to see a class of .243/6mm military rifles! The Brits almost adapted a 6mm in the 1950s, but lost out to our 7.62NATO.
I don't know what the proto 6mm case dimensions are like. From what I heard, it sounded like a fat .223, using a fast burning powder. If I stumble across any details, I'll letcha know. Might be good for your next novel!