badgerarms, the 6x49 is just my designation for the next-generation light rifle round. Its something I've been noodling over (despite having no expertise in the subject matter). Being more than 50 miles from Quantico, that makes me an expert!
(If you haven't heard this one before, the word "expert" comes from X, meaning unknown, and "spurt", meaning a drip under pressure.)
Basically the idea is to retain rather high muzzle velocities a la the 5.56x45, while allowing either heavier bullets (for improved penetration) or lower pressures (for better reliability). Same case base diameter as the 5.56 so 30 round mags aren't too huge. The dillemma is to go for a relatively untapered case (gives a more vertical stack in the magazine, thus makes mag design/manufacture easier) or go for taper (more reliable feeding perhaps).
Jeff, I can't argue with your logic, but I'm concerned that the Ordnance folks might yield to pressure from the "user community" to dock the barrel too much for a high-velocity rifle round. Theoretically I agree with you, as a 16" AR beats the hell out of any subgun I know of for up-close-n-personal problem solving, but I'd rather have a 12" barreled subgun than a 12" barreled carbine. That's my pessimistic thinking, anyways. Having recently read "Breakout" by Martin Russ, and a couple other memoirs, GIs seem to have loved the Thompson as long as the ranges were limited to 100 yards or so. The body armor issue is a real problem as you indicate.
JNewell, the "neither fish nor fowl" issue I think is the tricky thing with intermediate rifle rounds. That's why I split things into light/fast and heavy/fast, because with FMJ and especially AP ammo, velocity seems to be the critical thing, unless we go back to a heavy/wide/slow cartridge like the .45-70. So, for light rifles, we've got to give up bullet weight to retain the 3000 fps or so that seems to be needed for terminal effectiveness, otherwise the recoil becomes a problem. For MMGs (medium machine guns) at M60 size, as well as semi-auto sniper guns, then we can go to full power rounds because the gun weight is there to reduce recoil effects.