Its not an issue of being dry during the daytime, but all day AND ALL NIGHT LONG when you leave powder in the gun.
Sorry, but I think you have to dump or pull that load every night, and clean the gun DRY, if you want to have the gun go off. Of course, using a sub is very different than using Black Powder. The subs are based on oxidizers, like ascorbic acid. That begins to combine with moisture in the air no matter how " Dry " the air may feel to you. As the moisture mixes with the powder, the powder deterriorates.
If you insist on NOT dumping ( that means shooting) or pulling the loads( CO2, or Ball jag) every night, then about the only thing I can suggest you try is to put cellophane wrap( Saran Wrap for instance) on top of the nipple before you seat the cap. That should give a very good vapor seal to the nipple when the cap is placed on the nipple.
Then, at the muzzle, put a piece of cellophane over the muzzle before you load the patch and ball, or conical, or sabot, or whatever projectile you intend to use. Trim the cellophane when you trim the patch. The plastic wrap will provide a full vapor barrier between the powder and the muzzle of the gun regardless of the weather. It should also burn up fairly completely when the gun is fired. The piece over the nipple will also burn up or at least burn through when the cap fires, at most leaving a bit of wrap around the nipple, which can be easily removed.
I don't know what alcohol is going to do with that substitute powder you are using. It may break the powder down chemically. So, I do not recommend using alcohol when there is a load of that powder in the barrel( as you now do) UNTIL you find out from the manufacturer what alcohol will do to the powder.
If you were using Black Powder--- which you aren't--- you can use alcohol to evaporate moisture out of the flash channel, with no harm being done to the powder. Or if you are shooting a flintlock, you can put alcohol on a cleaning patch, and then cover the vent hole with part of the patch, folded virtical, while the rest of the patch is held by the frizzen when its closed. Of course, you won't have priming powder in the pan if you use this method, but it will keep moisture out of the main charge in the barrel, by evaporating any moisture in the barrel out of the barrel through the vent.
If you don't want to put plastic wrap down in your barrel, then cover the muzzle with plastic, oilcloth, well oiled or greased leather, a condom, or any other rubber cover that will prevent moisture from getting down the barrel while you are hunting. I have used electrician's tape, duct tape, masking tape, plastic wrap with a rubber band to hold it to the barrel, and the same kind of wrap using string to hold it to the barrel. I think the plastic wrap under the PRB will do a better job.
I have also considered using an OP wad, then a heavily greased patch just driven down to the OP wad, and then my regular patch and ball driven down on the greased patching, with the grease and the tight OP wad protecting the powder from moisture.
I am shooting Black Powder, only, so I can not tell you if any of these will work with a substitute powder. They should, as they represent a very real vapor barrier if used correctly.
