Ask veteran from which war? Vietnam?
I don't recall I have my M4 jammed even once when I spend 2 years in the Middle East. the Ma Deuce on top of my Humvee had jam more than I can remember. And I have fired probably more than 10,000 rounds on my M4.
IN the beginning, when they use M-16 base in Vietnam, it was a design fault that leads to the common jamming problem, this no longer exist as long as you clean your rifle frequently. I mean, any firearms jam if you don't properly maintain it.
As for 556 being weak rounds, that's a different opinion. 556 is more than enough to kill a person, but if you expect people to stop after taking a few rounds, I can tell you that it's doesn't happen that way, I personally see a guy took 5 AK round to the chest without Body Armor still stand up and shoot back, rifle round is not a blunt round, you don't stop unless it hit your bone, so either it cut thru your muscle or blood vessel and you bled to death, seldom you got shot with 1 or twice ( I got shot twice in Iraq by the way) and incapacitate you. That's across the board regardless of calibre.
But 556 is weak against armored target, that is true.