Again with the 6.5. Why don't you just marry 6.5mm since you love it so much?
I think a 6.5mm penetrator would have to have a larger parent bullet than .308. Note that this 6.5mm's penetrator is actually 4.5mm in diameter. You need a couple milimeters of sabot sheath. But they have .308 sabot rounds. Hell, the have .308 SLAP, HEI, and even explosive rounds, but the're not issued very often (probably due to ammunition costs and lack of a perceived need).
I can't find any data on the performance of these rounds, though. I don't think the .308 SLAP was ever fielded in any large quantity.
I think if you wanted a 6-6.5mm penetrator you'd have to have around an 8mm bore diameter. If the sabot sheath is too thin it'll break up in the barrel (which, I believe, was one of the problems wiht 7.62x51 SLAP).
Imagine, if we'll delve into a little science fiction, an 8mm cartridge with a non-metallic (and obviously non-reloadable) case. The catridge could have a recessed bullet like some of the designs they experimented with, making it a little thicker than a .308 round but also a little bit shorter. No matter, put it in a three or four column fat magazine to keep the capacity up. It fires a 6.5mm tungsten carbide bullet at some crazy high velocity and will probably zip through any currently issued ceramic plates without problems. THe high velocity and heavy bullet weight give it very long range, too, making it effective for heavy rifles, sharpshooter rifles, and machine guns. A wide variety of ammunition types can be devised, including full bore diameter heavy slugs for purposes that call for that.
Probably a bit on the big side for your average troopy though. So we scale it down. The goal is something with a cartridge size comparible to the current crop of intermediate rounds that can be used in a comparable, select-fire weapon. So we start with a 6.5-7mm parent bore diameter. The actual projectile is between 4.5 and 5.5mm in diameter, depending on how thin a sabot jacket we can get away with. The penetrator would obviously be smaller than the large "battle rifle" round, but with the lighter weight you could probably squeeze a little more velocity out of it. (Some good barrels are going to be necessary to prevent them from being burned up.) Recoil would be light and it'd be flat shooting, moreso than 5.56mm and with better penetration of both cover and armor.
Obviously, it wouldn't have any of 5.56mm's magical fragmentation that only happens in people and never happens in armor or cover (this allows 5.56mm to simultaenously out penetrate and penetrate less than .308. Like I said, it's magic. Just ask ARFCOM.) unless you designed a frangible round to do that (but you'd also have the option of using full-caliber jacketed hollow point rounds at lower velocities if better terminal ballistics was required.) But at the velocities possible, there's going to be a lot of kinetic trauma regardless of fragmentation, tumbling, or any of the other internet buzzwords.
Not sure how feasible any of this is, just thinking out loud. In a gunfight with these types of weapons, though, you'd better be very careful about what you choose to hide behind.
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