Cover the muzzle with something water proof. ON this short notice, plastic wrap and a rubber band, or electrician's tape will do.
I like to grease the bore AFTER I load the PRB down the barrel. That protects the bore from the ball forward to the muzzle from rusting. It also keeps moisture from condensing on the inside of the bore, where it can gather and run down to the ball and, possibly, soak its way through the patch to foul the powder. This is more a problem with rifled barrels, where sealing the grooves is more problematic, than with smoothbores.
Keep the muzzle pointed downward, and carry it under your armpit, so that whatever coat or poncho you wear to stay dry can cover the gun, too.
As to the back end of the gun, either plug the TH with a quill, or consider soaking a cleaning patch in alcohol, and then fold it into an "L" shape, laying it up against the barrel, on top of the EMPTY flash pan, so that one part of the patch covers the pan, and the other covers the TH. The evaporation of the alcohol will keep any moisture that might get into the barrel and powder OUT. Just close the frizzen on top of that lower part of the cleaning patch. Take a small bottle of alcohol with you, so you can freshen the patch if it dries out in a couple of hours.
This method will require you to remove the patch, and then prime the pan when you hear or see a deer coming, but unless some other hunters is pushing a deer, deer usually move very slow and cautiously in the rain. You should have plenty of time to prime your lock.
You can also use a Cow's Knee to cover the whole lock, but unless you already have it, its a bit too late to get now.
The Rain robs Deer of their ability to Hear well, and disturbs their ability to Smell ( it washes the air of scents). Add the winds that normally come with these storm fronts and you get NOISE, as well as lots of air movement that doesn't give deer much time to smell anything"! Under these conditions, deer are forced to use their Eyes Only for self defense, and they are Not used to this kind of sensory limitation. All these reasons cause deer to move cautiously, forcing them to stop and look, and try to smell the air before entering an open area. ( About 95% of our human sensory perception is visual, unless you do regular exercises to increase your ability to smell, hear, taste, and touch.)
[Hint: Put a blind fold on. Then sit and listen, and smell. Have a friend map a course, with ropes strung from tree to tree. Then walk that course blind folded, identifying plants, and trees by their feel, smell, and even their taste.] You will find these exercises will not only enhance your sensory abilities with your other senses, but they will also enhance your ability to SEE MORE. :hmm:
Don't move fast, or loudly. Remember that most wild animals "Listen" with their feet, too. If you stomp through the area, the vibrations in the ground made by your feet will be felt by animals a long way from you. Walk lightly, and slowly. Use the same Rhythm of walking that you hear deer use- same speed, same pattern of steps, same length of stopping, before moving again. Stick to game trails- Don't "bust brush". Watch out for making noise above ground when your clothes brush against branches. Listen to the alarm calls of other small animals. Squirrels, and birds often will be the first to spot you, and "announce your presence to everyone". Stand absolutely still, until the calls stop, and then double that amount of time before moving again. Move only when the wind is blowing, and then only as far as the wind moves a tree branch or the grass in front of you. Stop when the wind stops. never stand upright, unless you are next to a large tree, that will mask your upper body movement. Plan your movements to use the game trails that run from one large tree to the next.
Its not 100% true, but most deer that are feeding, with their heads down, wiggle their tails back and forth before lifting their heads up to take a look around. Watch for that, and STOP. Get down low so you are not silhouetted against the sky. Wait for the deer to put its head down again, to move.
All of this is made more difficult in the "Wet", and particular concern has to be given to keeping the action dry. I recommend taking several towels, or paper towels, to use to wipe down the flint, top and bottom jaws, and frizzen, both the top and UNDERSIDE of the frizzen, and then the pan of course, before priming it. Prime with the same powder you use in the barrel. Leave the $fg powder home. The coarser 3Fg or 2Fg powder will stay dry longer, and will fire just as well, if just a few milliseconds slower, than 4Fg. Most of us can't hear the difference.
With the coarser powder, I can wait up to 20 minutes before checking the priming powder to see if its beginning to develop a "crust". If that is present, I remove and clean the pan, dry the entire action, and then re-prime. I cover the action with my hand and hold the gun as close to my body as possible as I move in a crouch. Let the muzzle point the path in front of you. It will keep you on the game trails.
Coat your gun, both stock and barrel, with a good furniture wax( NOT AUTO wax) to protect both from the rain. If you use a paste wax, force as much wax down between the barrel and stock as possible, to act as a sealer for the barrel mortise. That keeps water from getting UNDER your barrel and rusting it.
Have a successful hunt. :hatsoff: