It would help to know what model of Lyman Percussion gun you have as there are different models out there that do different things. To know the model of the rifle is to know the twist of the barrel and that will tell me what might work in your rifle and what might not.

But first lets address the moisture problem you experience. A percussion cap rifle should stay as nice and dry as your center fire rifles when treated right. First thing I would do is go to RMC Sports.com and get a new nipple for that rifle. The nipples that come on a Lyman Rifle for some reason just do not like to work properly. The RMC work great.

Next we have to prepare that rifle to shoot. If you do not prepare that rifle it will misfire rain or shine. So before you even load it to go hunting, first take a patch. Apply some isopropyl alcohol to it. Just damp, don't saturate it. Now swab all the oil out of the bore. Oil believe it or not can kill a charge as fast as water. After you have swabbed the oil out of the bore, take a dry patch and patch the alcohol out of the bore.

Now put another dry patch on the loading jag and push that to the bottom of the barrel. Put a cap on the nipple and fire that cap. Pull the patch and check it for burn marks. If there are no burn marks, push that patch back down to the bottom and do it all over again. Again, check for burn marks. What you are seeing is if fire is making it through the nipple, then the drum, into the fire channel. When you see the burn mark you are set to load.

Since it is a percussion cap rifle, you know pellets do not work. So you load loose powder. To shoot roundball, load 80 grains and try that. Then take your patch with a good lube on it. Set the roundball in the center of the patch and load that into the barrel of the rifle.

Make sure you seat that ball firmly on the powder charge. Conical bullets do not require a patch. Only lube. I use a .018 patch for my roundball rifles. I want the patch to be pure cotton. I use a moosemilk lube that I make at home.

If you are shooting conical bullets, just dump the powder and then the conical bullet goes next. If your shooting a Lyman Great Plains rifle, they are a roundball rifle. If it is a Lyman Great Plains Hunter, that is a conical rifle. If it is a Lyman Trade rifle it will shoot either one. If it is the deer stalker they also shoot either one.