In 13 years of shooting the AR platform in the civvie world (Armalite, Bushmaster, Colt) and after 6 years in an infantry unit in the ARNG (Colt, FN), I have to say that I've never had a jam with live rounds. Ever. Blanks can suck a goat, but blanks are blanks.
That being said, I have not been in combat, nor subjected a rifle to combat conditions. Therefore, I can only conclude that the AR platform is reliable, when well-maintained.
I have never encountered a bad magazine, either. The Circle-K mags that I was issued (and I ended up with 13 of them, don't ask me how), are all reliable.
I have seen the M16A2 and the M4 choke on live ammo, but it wasn't my rifle. Even still, such events seem rare to me.