I hunt in the SE part of Al. It often rains during our special 5 days of ML I hunted last week during the rain as a matter of fact. I don't try to keep my rifles out of the rain. I use the fingers of "surgical" gloves or a balloon strectched over the muzzle. I have had no misfires. Also I have test fired my ML's (Savage 10 ML and CVA 50 Cal. Optima ) at the range to satisfy myself that there is no adverse effect of ahving the covers on the muzzle. I have found zero effect on accuracy or drop out as far as my shooting range goes .... 150 yards.
I simply wipe down my rifles when I get back in to camp and will clean them thoroughly when I get back in after taking a shot (s). I have had zero rust issues.