A) The story I learned was that the patches over the gun ports kept debris out, and also was an indicator of wether or not the weapons had actually operated or not helping the armors to trouble shoot a/c coming back from combat.
Browning machine guns operate on the " blow back " principle,weapon is cocked,round in the chamber, breach CLOSED. Trigger is pulled, the internal hammer strikes the firing pin located in the breach block, and fires the weapon. The pressure from the fired round opens the breach block, and strips out the empty case.The recoil spring forces the breach block forward, chambers another round, CLOSES the breach block, and the weapon fires again....as long as the trigger is pulled, it will continue to fire. So, how the cold air got into an open breach is a little impossible.... cold air getting up into the receiver thru the spent case chute is entirely possible, hence, taping off the openings.
Scott J