The 6.8 is about the same size as the 308 but has 80,000 lbs pressure vs 308 or 7.62x51mm. It can penetrate body armor to 600 yards. This is why they chose this cartridge. It was the only one that could penetrate body armor at that distance. This will take time and will go to the army first. National guard units will still use the 556 for several years to come, like 5-10. Don't know if the Marines or other units will adapt it, but probably will over time. The 6.8 will make all current body armor obsolete.
