There's a little dingus called a cap guard, I think, that's a section of soft, pink tubing that goes over the nipple and cap to seal that end. A finger cot or small balloon over the muzzle will seal that. It's also a good idea to put some paste wax, bullet lube, or grease over the base of the nipple to keep water from pooling there and seeping inside.
I killed a doe in a windy wintry mix a couple years ago using the cap guard and by keeping the muzzle down. Conditions could hardly have been worse for a ML.
