Lets start over: how do you design a GPC?

[stanc]

So how small a case can we make that still provides the same performance as the 7.62 NATO?

That might best be answered by looking at cartridges that are actually being developed by private individuals for this purpose:

- Cris Murray created the 7x46 UIAC and 6.86x46 UIAC, based on the 0.44" diameter 7.62x45 Czech case. Cartridge OAL is ~2.6" IIRC.

- RRBud, a member of Tony's MG&A forum, has made the .270 Sidewinder (6.8x50), based on a strengthened .30 Remington (0.42" diameter) case. Cartridge OAL is 2.8" IIRC.

- Harrison Beene, another MG&A forum member, plans to develop 6.5mm and 6.8mm rounds based on (0.45" diameter) 6.5 Carcano brass. Case length and cartridge OAL to be determined.

Judging by these examples, it looks like cartridge OAL will probably be equal to that of 7.62 NATO, or somewhat shorter, depending on variables such as case diameter, bullet diameter, bullet weight, muzzle velocity, etc.

And if we find that caliber and case, how should the bullet be made in order to be compared to the M80 cartridge?

My opinion is that a comparison of relative performance requires that a lead core FMJ in the candidate caliber(s) be tested against the lead core 7.62 M80. Improved 6.5/6.8/7mm ball rounds could/should also be tested, if available, to give a more complete picture of performance capability.