Blain
Do please tell about your combat experience with the M-14 that would lead you to make this statement. While you're at it, tell about the times the M-16 let you down in combat, too.
Okay, I'm not Blain, and I have done no combat, but I can tell you about some range experience, almost all in deserts and perhaps 10% in dusty conditions.
3,000+ rounds through a Springfield-receiver GI parts M1A I built up, maybe 2,000 rounds through a Colt HBAR, 1,000 or so through a Valmet M76 and miscellaeous rounds through a pair of Imbel-receiver FAL parts guns and various buddies' ARs of various makes.
Even under these mere range sessions and a long-walk bunny hunt with the Valmet, they have all jammed, but never once because of dust. The M1A was mishandled and fired with its op rod outside the receiver channel. Ugly jam but easily fixed with a bit of muscle to the system.
With a bad mag (missing 3/8-inch of the front side of one feed lip), the M1A once had a smokestack feed jam. Bolt stopped forward, with the live round pointing out of the recever group. Easily cleared.
The Valmet had chronic magazine problems, now almost all fixed (Galils won't work well). Also had many problems with reloads getting case separations, not at the head, but halfway up. All easy to clear. Placed well in a 3-gun match racking rounds all over the field, from "broken case extractors" which were the next fed round, to tipping the receiver to dump rounds that sunk the bullets all the way in when the rounds smacked the barrel below the chamber (Galil mag did that the most).
FAL--adjust the gas system per instructions and you'll need to fiddle again with light loads that still function just fine in the M1A. Otherwise, not much experience.
ARs: I've twice seen a bad mag send a round up into the channel in the underside of the charging handle. The bolt lugs then crammed the round up into the space around the rear end of the gas tube. Can you say totally unclearable jam?
The ARs I personally fired were all kept with minimal lube, and collected very little dust. Half of them have either mysteriously failed to fully chamber a round or skipped feeding a round from the mag at least once, despite being fired less than 200 rounds between cleanings. None of these were from bulged or bent cases, either. Others I've observed on the firing line years ago at HP matches seemed to mysteriously misfeed as frequently as M1s (NOT M1As), and that's not counting the reloaders who forgot to charge their cases.
From the credible-sounding reports I hear, the ARs frequently keep on ticking for very long times. But it also sounds like when they choke, they choke badly. I'm all for some objective side-by-side testing under the talcum-powder sand conditions of Iraq, using various states of lube/no lube and types of lube. Because so many spaces in the AR receiver give the dust no place to go, I'm predicting the M14, run almost dry, will perform better than anything else out there, in strings of 500 rounds between cleanings.
The Valmet with decent mags will come in a very close second, or will be equally jam-free at zero. But in .223, it might rip a casehead right off when the chamber gets a bit dirty. Extraction is a bit...vigorous.