I get flamed when I criticize the 6.8SPC. Awhile back I suggested the 6.8 was the wrong direction and that the correct direction would be to put a heavier bullet in the 223 and then up the chamber pressure and the muzzle velocity by use of faster burning powder and/or more powerful primers. It wouldn’t hurt to make it a boat tail bullet either, since, I’m pretty sure the 223 is not commonly a boat tail type bullet. If brass starts to split or bulge, make it thicker walled, or switch to aluminum casings. Whatever it takes.
The SPEC OPS foks have done just that-the Sierra 77 grain matchking (now called open tip, as hollow point is not technically correct nor geneva convention PC) and it does something like 2750-2800 out of an M16A2, somewhat less from a carbine. I beleive they call it the Mk 262 mod1 (Navy has the lead on it). This is the equivalent of what wins National Match competitions out of AR15A2s (200-600 yard).
Some rumors indicate that the rest of the Army may be interested in using it as well.
The 55 grain M193 and the 62 grain M855 (SS109) NATO 556 mm bullets are indeed boat tailed bullets.
The cartridge cases of the 556 are tough enough for our purposes-adding thickness reduces capacity-negating any improvement-however, mil spec brass is somewhat heavier than civilian counterparts.
Side note-commom BTHP type bullets perform very much like FMJ bullets in flesh-they do not expand like hunting soft point bullets, rather, they tumble and often break in to at the cannelure.