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There may have been a bit of "fudging" going on the US tests to make the US rifle come out on top. Certainly even when comparing the early AR-15 to the M-14 some of the tests were less than fair or balanced.
While the M-14 is a decent rifle a few US generals were way
too wedded to the "concept" and not the reality of the weapon. It was
supposed to replace the M1 rifle, the M1 carbine, the M3 grease gun and the BAR. It was
supposed to weigh only about 7 1/2 pounds and yet fire a round just as powerful as the .30-06 M2 ball. New powders meant the cartridge case could be smaller yet give the same performance. Just becasue you are a general does not mean you can repeal the laws of physics howver
For instance ALL military M-14s have the "ability" to fire full automatic, they just have the selector switch removed. Given the amount of drop in the stock (distance from the axis of the barrel and the pivot point on the shoulder) and it's weight anything after the 2nd round (and that one was going high) was heading for the sky. One plan was for the squad leader to carry the selector switches in his shirt pocket and issue them out in case of an
emergency 

Civilian M-14 clones do not have the hole in the side of the receiver to take the selector switch.
The M-14 had a fixed gas system, some FNs have a two position or variable system were if the action is sluggish for any reason more gas can be admitted to the cylinder for a bit more
omph. The Bren had a similar system pre-war and may not have been the first with that ability.
A bit of playing with the rifles in cold weather to balance the amount of gas needed ( size of hole in the regulator) and the buffer strength should sort out most cold weather problems, there is very little in the inherent design of any gun that makes them more sensitive to cold than another design (except maybe tight tolerances?). If you can get a recoil operated gun to function in cold weather a gas gun should not be a problem. Recoil operated guns having a more "fixed" amount of power to work with since they balance moving part weights and springs. Not much adjustment on an already manufactured gun although a lot of tweaking can be done before production ( or the introduction of a MK II)