The M855A1 EPR round in the 1/7 M4 is as effective at putting bad guys down with authority as anything the M16/M4 has ever been chambered for, including the original 55-gr round thru a 1/14 bbl. If you don’t believe me, listen to Kevin Owens, former Tier-1 anti-terrorism with the Irish Rangers and 18 years as a Green Beret with SOCOM (he’s killed more people than cancer):
https://www.youtube.com/watch?v=84KBHdpKB1c&t=314s
But the 6.8 SPC is a big step forward for CQB because it isn’t bullet-sensitive, whereas the AR15 platform has proved itself extremely so (Big Army’s fault, not Eugene Stoner’s). However, the 6.8’s case head is 0.44” larger than the .223’s so there is a small loss of magazine capacity.
And the M4 is a stone-reliable platform thanks to millions of unpaid beta-testers over the last 60 years. Cheap AR-15s now are more reliable than guns Colt built in 1963 because of the lessons learned in more than half a century of manufacture. It would be a shame to bin it because it can’t drive a 130-gr pill at 3000 fps.
OOPS ... that should have been 0.044" larger. I moved the decimal. Sorry 'bout that.