That is absolutely true, and thank you for not tearing me down because of this newly found passion that has entirely engrossed me. But I hear you guys, I'm wanting something that doesn't exist because if it did, you'd hear about it more often. Thank you Tony.
No problem, if you would've marched in here pretending to know everything like some guys do this would be more entertaining though!
I'll try to do a quick breakdown of some of the basic problems with what you're asking for, without writing a short book.
Regarding 308 or 6.5CM, they're in a bigger cartridge family than the other stuff listed, 300BO, 223, 6.5G etc., and not only require an AR10 sized platform (bigger, heavier, less parts commonality, all bad for PDW stuff) they also use a lot more powder and will always have more recoil. Because they use more powder you want the gas system further from the chamber but in a 12" or shorter barrel the gas system will be short and BCG function will be violent and recoil harsh. Probably very hard on suppressors also and not as quiet. Not suited to subsonic loads in a gas gun because when you consider that a full power load is burning around 42 grains of powder and a sub maybe 8 grains you'll understand that its impossible to optimize a gas system for one that will function or be safe at the other end of the spectrum.
You can get a 300BO to run subs and supers because a full power load might burn 20 grains while a sub maybe 7-9, so the difference at either end isn't nearly as extreme.
The difference in ballistcs between 6.5G and 6.8 is splitting hairs under 500yds, both of them will do what you want. But understand that everything is a comprise. The short barrels that you want will give you less velocity, which means more drop and wind drift. The gun may still be capable of MOA at 500 but the lower velocity will make your wind calls and range estimate accuracy even more crucial.
Don't forget that a decent amount of magnification really helps to shoot small groups at 500, and a big heavy scope kinda defeats the purpose of a small light gun. Not saying you can't do it, just saying its another compromise and you could end up with a gun thats just barely OK at most things and not great at any one thing.
