Roberts wrote in his presentation that testing to develop the 6.8mm looked at bullets including 6mm, 6.5mm, 6.8mm, 7mm and 7.62mm.
The 6.8mm offered the best combination of “combat accuracy, reliability, and terminal performance for zero to 500-yard engagements in an M4-sized package.”
I don't believe the good Dr. Roberts for a second, either .270 is a magical bore, or it's just not true. Maybe the specific projectiles tested performed the best in their test according to Dr. Roberts' evaluation of the results. But nothing magically "optimal" about 6.8 vs 6.5 or 7mm diameter bullets.
6.8 SPc would be a bad joke for a general issue cartridge to replace the 5.56, so I assume they have something totally different up their sleeve, like a new CT or polymer-cased round.
