New Army "Caliber Configuration Study"

[nincomp]

With the adoption of a GPC we'd see a LMG and MMG system. The new LMG would fall somewhere in between the current LMG and MMG. Exceeding the requirements of the current LMG and not quite being suited for all the requirements of the current MMG. The new MMG would meet all the requirements of the current 50 cal HMG while being able to meet the mission requirements of the current MMG that the new LMG isn't AS suited towards (Vehicle Mount).

I realized that I did not address this part of your post. As I understand it, a GPC would only replace the MMG for dismounted soldiers. The 7.62x51 would stay in service for mounted installations, at least for a while (since we already have a large number of 7.62 MG's and ammunition). The 7.62 might eventually be replaced by by something more powerful, like a .338, but that is a separate argument.
One major issue will be the performance of the 7.62 if the military insists on lead-free ammo. The chambers for the 7.62x51 (and the 5.56x45, for that matter) were designed for lead/copper bullets which do not have long ogives sticking out of the case. This not only prevents longer, more aerodynamic bullets, but it means that more of the longer lead-free bullet will need to be crammed into the case, reducing the volume for powder. *

It would not surprise me if MG terminology and use gets more confusing, but maybe not. This is what I see likely:
LMG - GPC: carried by dismounted troops
MMG - 7.62x51: mounted MMGs Note: might eventually be phased out, possibly by a .338.
HMG - .50 Cal**
GPMG - ? Depends on your definition. Possibly none.
"Holy Crap" - minigun

As to 7.62x51: It would not qualify as a GPC, given Tony Williams' definition. When it was introduced, "Universal Cartridge" would have been closer. To be more accurate, "Universal Cartridge except for the .50 BMG, for Heaven's Sake," but that is a bit wordy.


* If a new lead-free bullet is to weigh nearly the same as M80, it will be longer. The most likely materials, steel and copper, are less dense that lead. Search for images of "M80A1" and look at the 147grain variant. For 5.56x45, see the M855A1

** Sometimes, bigger is better (despite what I tell my Wife)