OIL & Grit
I can tell you what oil mixed with fine sand and grit does to an FNC1A1 7.62mm ...... binds it up PDQ to the point where you needed a bayonet to open the folding cocking handle.
Crawling through the weeds in the Wainright Training area (or Shilo), bouncing around in the back of canvas topped vehicles, playing in the sand box, etc., etc. grit will find every possible opening and crevice to creep into and bind with the requisite 'thin film of oil' the army required.
Once you got it cocked, it worked. Never heard a Brit complain about their SLR version with the cross-slotted breechblock carrier, and they served in some pretty dry, dusty terrain in the Middle East. The cocking handle on theirs stuck out where you could get it.
Magazines accumulated an incredible amount of dust and dirt which found it's way up and into the breech when the rifle was fired. Always a good way to catch out a soldier was to strip a magazine ......
I keep my Garands greased on the points recommended. If they did that in the Pacific, North Africa, Sicily and Italy, they must have had a problem, but they didn't have a rep for binding up.