6 or 6.5
Actually, uglygun, 6mm BCs optimize at around 100 grains (in lead spitzer boattail bullets - might be heavier with a steel core), and 6.5s at 140/142 gr, so if'n you need a lighter (100 gr) bullet to get the vels for what you're trying to do, trajectory-wise, then 6mm makes more sense for very long ranges. But, having said that, I agree that 6.5 *IS* the better all-purpose round (esp. for the fabled all-purpose ideal military round), in a 140 gr config - in a case somewhere in size between the 6.5 PPC and .260 Rem, closer to the .260 Rem - you should get vels of 2400-2600 or so from a standard barrel in this theoretical round (you get 2700 with 140 from a .260 Rem), and have more penetrative power and energy at range than the 6mm bullet, with not much more bullet drop, and certainly much less drop than a 150-168 gr .308 bullet. Gabe, I think you're right - that case sound like an interesting compromise. Oh yeah mightn't the 6mm might be more accurate than the 6.5s as I understand that 6mm makes for an inherently accurate cartridge? The 6mm Rem BR is a slightly fatter and longer case than the PPC (same width as a .308), giving more vel (around 3000 with an 80 gr bullet), and Glocker mentioned "controllable" - this should be a controllable round to shoot in full-auto, *if* that's what you meant Glocker. You'd have to have a case the size of .243 or bigger to get 3000 with 105 and that's too much powder for what I understand is the parameter of your question.
PS. The 6mm Rem BR might be a good starting point for your inquiry, but I think the OAL will be out of spec for that cartridge with 105-115 bullets - but voila, just make your own "wildcat" with a moderately longer OAL (and possibly a slightly longer case length) - but same basic cartridge. In fact, this 6mm Badger - is it named after Badger Arms of THR fame?
??
The 6x47mm is a .222 Rem necked up to 6mm. It shows vels around 2800 with 87 grain bullets. It's a narrower case head (.378, even smaller than a 7.62x39) than the BR so you could fit a LOT more in a mag, but slightly less vels than a 6mm BR.
I guess it just keeps coming down to trade-offs, and how you rank the relative importance of your goals for the cartridge: Performance at 300 yards? 500? 600? Controllable full-auto? Mag capacity? Etc...I'm fair to partly dangerous when I spout off, now that I have Speer and Hornady and Lyman manuals, and "The Handloader's Manual of Cartridge Conversions" - lol - the latter makes my head spin.