The M1/M14 types are not bad in the field, as real world testing has shown. Dirt can get in easily, but comes out easy also, something not true of rifles with closed in receivers.
One piece of info may be of interest. Just before one of their wars, Israel rearmed their front line troops, replacing reworked K.98k's with brand new FAL's. Their troops, as troops will do, either rode into battle on tanks or followed the tanks to take advantage of the cover. Naturally, they were swallowed in clouds of dust.
When it came time to open the ball, the Israeli troops took aim and fired. Some FAL's wouldn't go off, others fired the one round in the pipe and quit.
So the Israelis, in great secrecy, pulled back the FAL's and re-issued the (you guessed it) K.98k to front line troops. Then, with typical Israeli "hutzpah", they gave the FAL's to reserve units and told the press that they now had so many of the new rifles that they could even arm reserves with them.
The K.98k didn't care about sand, dirt, mud, cold, heat or anything else; it just went bang when needed.
Jim