Amount you can carry
Fighting between the big-round and small-round groups reached a peak in the early 1960s, when test after test showed the .223 Remington (M193 5.56×45mm) cartridge fired from the AR-15 allowed an eight-soldier unit to outgun an 11-soldier unit armed with M14s at ranges closer than 300 meters. U.S. troops were able to carry more than twice as much 5.56×45mm ammunition as 7.62×51mm NATO for the same weight, which allowed them an advantage against a typical NVA unit armed with Type 56-1s.
https://en.wikipedia.org/wiki/7.62%C3%9751mm_NATO#Adoption_in_battle_rifles
Interesting chart there too about how much ammo you can carry in a 10kg load of magazines.
260 rounds of 762 NATO
620 rounds of 556
360 rounds of 762 Soviet.
Long Range Effective Accuracy
On the other hand, once you get out to like 300 yards/meters, lighter rounds will get tossed around by the wind, but personally I was having success with very heavy 85gr Open-Tip Match 5.56. But that's premium stuff, and standard 55gr and 62gr projectiles were very hit or miss shooting prone at a 8-inch target.
Recoil and Quickness of Follow-up Shots
.308 out of an AR-10 feels like twice the recoil of 5.56 out of an AR-15. It's not fun. The gun weight too feels like twice as much. If I'm lugging something around I know which I'd prefer.
What are your thoughts? Also, is the new larger US military rifle/caliber contract won by SIG Sauer dead in the water? I think it is, except maybe for specialized roles like DMRs.
one of the reasons I hate Americans is how they want to be special and have a bunch of different measurements.
.45 (inch) ACP, but will use 9mm. .223 and 5.56 are nearly identical but if you mix up the two in the wrong gun, say goodbye to your hands. JUST BE NORMAL
Nah, it's been decades since swapping .223 and 5.56 was a problem. Any .223 or 5.56 gun made since the 90s will handle either just fine. You have to be a little bit more careful with 7.62x51 NATO guns because .308 is loaded hotter and can damage a 7.62x51 NATO gun, but again in modern guns it's unlikely to cause a catastrophic failure.