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.
"Stopping power" is another consideration I forgot to mention. I agree the difference is exaggerated. If you need to stop a guy right away, you need to hit the heart/aorta or central nervous system. Smaller caliber makes it easier and offers more chances to followup if you don't score a critical hit. There's recent Ukraine footage of a trench raid and the camera guy has no trouble downing people in plate carriers and helmets with what looks like a 5.56 rifle.
also the dogmatism about "stopping" being turning a guy into hamburger, while i suspect a good majority of folks are out of action with less than (immediately) fatal injuries
You're right about that, plus on the extremely morbid end a dead guy takes no enemy resources while a wounded guy does.
Yeah, stopping power is fudd bs. All that matters is that the round can penetrate in to the vitals at whatever distance you're shooting. Pragmatically everyone just needs to stop worrying about it - if you're carrying a rifle you're most likely going to be killed with artillery or machine gun fire before you even get a chance to deploy the hyper-optimized rifle you can't get parts or ammo for, so just relax, get an AR without any weird internals, and use M193
Aside from a few very recent and expensive helmets with heavy up-armor plates helmets won't stop rifle fire of any caliber. You might get exceptions at long ranges and funny angles but within 100m the helmet isn't going to stop a rifle. It's primarily to protect you from shrapnel, spall, and artillery fragments. Plates are cool but they only cover part of your chest and while getting hit in the guts or pelvis might not kill you quite as instantly you're still most likely done with your fighting days, assuming you don't bleed out.