Personally, if you're going for "badass," I'd want to see something along the lines of a bullpup SLR, a rifle noted for its power and accuracy. This strikes me as a squad support weapon, where you'd need standard riflemen to back up the heavy weapons personnel who'd be toting these weapons. Furthermore, you'd assumably need to issue sidearms to anyone employing these weapons as they might be quite hazardous in very close range combat situations. Though you might be using less ammuntion, your ammunition is also going to be heavier and bulkier - and if you're carrying a sidearm, your normal ammunition supply has a separate contender to throw into consideration. Also, the ammunition is bound to be more expensive regardless, so "saving money" is more likely than not going to be a myth with this thing.
Looking at the numbers, it also makes sense that this would be a support weapon - 12,500 is not a lot given the size of the US Army and Marines. All we're doing here is just reverting to the more specialized squad orgainizations seen during WWII, etc., rather than the seemingly homogenous "assault rifle" squads of late.