6.8mm works out to .268 caliber (.270).
https://en.wikipedia.org/wiki/6.8mm_Remington_SPC
Based on the .30 Remington cartridge,[7] it is midway between the 5.56×45mm NATO and 7.62×51mm NATO in bore diameter. It uses the same diameter bullet (usually not the same mass) as the .270 Winchester hunting cartridge.
.270 Winchester uses .277 caliber bullets.
Mate, when push comes to shove it’s always a compromise between projectile weight, velocity, cartidge size/weight etc. I carried an M14, 18 mags plus one in the rifle, 6 grenades, 1 Claymore, rats, deet, socks, etc., and didn’t bat an eye. But then again at 6’3” and 210 lb. I wasn’t effin bothered. Give me the biggest bullet mass/velocity you got and I’m a happy jack. 200 rounds a day on the course was nothing. What’s a bruised shoulder to proficiency. 24/365 and happy.