If someone wishes to hunt big game with an AR15 platform, the 6.8 SPC and 6.5 Grendel will help the hunter meet legal requirements. When hunting elk or moose in Colorado, a bullet must have a minimum caliber of 6mm, a minimum energy of 1,000 foot-pounds at 100 yards and weigh at least 85 grains. 223-caliber ammunition is not permitted for big game hunting.
For military use, there is a tradeoff. While the 6.8 SPC and 6.5 Grendel have advantages in energy, these new cartridges have larger diameters, are heavier and fewer rounds can be carried in the same volume magazine or ammo box. More fire discipline may be required when carrying fewer rounds. Unless the newer cartridges provide a clear advantage, it is likely that 5.56x45mm will remain the NATO standard.
I see the 5.56 remaining the NATO standard, but think the AR/M 16s days are numbered.
I don't think the .223 will do anything the 6.8 will do, but I also don't think the 6.8 will do anything the 5.56 will do. They're two different cartridges. In a Varmint configuration, the AR 5.56/223 is capable of whacking p-dogs out to long ranges with little recoil. I don't think the 6.8 will do that. Also, while the 6.8 is a good deer cartridge, (if the range isn't too great) so is the .223 (if the deer isn't too large.)
Were I going to buy an AR type rifle, which I won't since I've had one since 1972, I'd without hesitation get a .223. If you only have one, that's the rifle to get. If you already have a /223, maybe a 6.8 for reasons still unclear to me.
