Okay, WHY are we limiting case length to 39mm? The Original Ar-15 cartridge ran 45mm in length. I'm not sure what the max C.O.L. is, that can be run through one, but I'll wager that it'll be very close to the same, whether the developed cartridge launches a .22 or .35 caliber projectile. Tossing away available cartridge length surrenders case capacity for no real gain. Case capacity translates to higher velocity/lower pressure, which is something we should not give up too willingly.
Yes, it IS true that it would be easier to adapt existing species of cartridge to the task, but if the mission is to "create NEW", then LET US "create NEW". To base the firearm on the "mighty MATTEL" strain of rifles already concedes more to convenience than one might consider prudent.