The only other issue I see is with the increased proliferation of body armor the 5.56 isn't cutting it any more. That said, I don't think the 7.62 does either. I've personally met people in theater that survived 7 shots to their chest plates with zero penetration by 7.62x39. The ceramic plates we wear are meant/designed for 3. And soft tissue ballistics will show that from a 20" barrel the 5.56 will do more internal damage.
I think that's where the 6.8 comes in. It is designed specifically for shorter barrels. The 5.56 is most effective when fired through a 20" barrel. The issue being that the M4 only has a 14" barrel.
Obviously for real stopping power and penetration a .308/7.62nato will take the cake. The issue there being ammo weight, weapon weight, barrel length, and recoil.
That's why the mid-range rounds like 5.56 and 7.62x39 were developed. Matt knows more about ammo than I do so I'll wait for him to chime in and school me.
The 5.56mm is dated for stopping power, but the good old .308 is large a lot of recoil and heavy.
That is why I am so intrigued with the .300 BLK (7.62x35mm) same ballistic and better at the 300 meter mark than the russian 7.62x39. Uses all the same hardware other than the barrel as a 5.56mm.
I am about ready to convert the PUNISHER 5.56 in to a .300 black out. I like the bigger projectile floating down stream.
I saw a ballistics chart of the difference with 16" and 20" barrel and the muzzle velocity only increased 100 fps with M855 rounds. So there is not a lot of advantages of running a longer barrel.
6.8spc is a larger bullet but from what I read does not of any more advantage for distance than the 5.56mm. What I did here is it loses a lot at 300 meters and beyond. Also the changes to the AR for it to work is a lot more than .300 BLK and the mags are pricy and only hold 25-26 rds.
Here is some cool charts of the .300 BLK compared to 5.56mm and 7.62x39; (muzzle energy can be more important than velocity a feather and a brick both traveling at 1000fps the feather wont hurt so much the brick.............yes)
9-Inch AAC Blackout Ballistics
@ Muzzle:
5.56mm - M855 2558 ft/s, 901 ft-lb
300 BLK - 155 gr SMK 1785 ft/s, 1096 ft-lb (22% over 5.56mm)
7.62x39mm - 123 gr* 2022 ft/s. 1117 ft-lb
6.8 SPC - Hornady TAP 110* 2157 ft/s. 1136 ft-lb
300 BLK - 123gr MC 2130 ft/s, 1238 ft-lb (37% over 5.56mm, 9% over 6.8 SPC TAP 110)
16-Inch AAC Blackout Ballistics
@ Muzzle:
5.56mm - M855 2938 ft/s, 1188 ft-lb
300 BLK, 155 gr SMK 1940 ft/s, 1294 ft-lb (9% over 5.56mm)
7.62x39mm Win - 123 gr SP 2245 ft/s, 1377 ft-lb
300 BLK - 123 gr MC 2315 ft/s, 1462 ft-lb (23% over 5.56mm, 6% over Win 123 gr 7.62x39mm)
Remington 6.8mm SPC - 115v gr MC 2400 ft/s. 1471 ft-lb (1% over 300 BLK)
6.8 SPC - Hornady TAP 110 2540 ft/s, 1576 ft-lb (8% over 300 BLK)
