When you put any kind of lube down the barrel- including bore butter- Always first try to get as much out with a cleaning patch, and then run a patch dripping with alcohol down it the barrel to dissolve what remains. You want it wet enough that alcohol and oils or liquified grease run out the drum and nipple hole. Always remove the nipple when doing this kind of cleaning. Drop it in a small container with alcohol in it to clean it out, too. Use a pipe cleaner dipped in alcohol to liberally wash and clean out the flash channel or drum. Dry the barrel again with a cleaning patch or two, to reduce the time it takes for the alcohol to evaporate, and absorb some of the oils and liquified grease. If in doubt, repeat the alcohol process. Let the gun sit while you dry and check the nipple to see it has no obstructions. I blow throw my nipples to do this. The wire stands by in case there is an obstruction. I always put a drop of oil on the threads of the nipple and in the patent breech, or drum threads, before I screw the nipple back in the gun. By the time that is done, the alcohol is evaporated, and the gun is clean and dry.
Now its ready to load powder into the barrel. If I am hunting in damp conditions, after I seat the PRB on the powder, I will run a cleaning patch liberally coated in bore butter down the barrel to lube the lands and grooves in front of the PRB, which is also lubed. That leaves the drum, flash chaneel, and nipple clean and dry, with powder in the chamber, and, perhaps, in the flash channel or drum. Loaded that way, the powder stays dry and the gun ready to fire all day long. I have hunted from before suup, to after sundown, in mist, fog, and rain, and had my gun fire at the end of the day when I wanted to empty the gun for safe, and, at that time, legal transportation.
There is nothing wrong with using bore butter to lube the barrel during storage. Other than Ballistol, and olive oil, it is one of the best preservatives. But, with all of these, you have to remove them from the gun before loading that first charge. Usually, firing a couple of caps is not good enough to get the job done. I have burned gunk out of the chamber by putting a charge of powder down the barrel and then firing a cap to burn the powder. Done fast enough, the powder ignites before the gunk has a chance to spoil it all. But, then you have to clean ous the greasy gunk. I found using alcohol- rubbing or denatured- was a faster, and more sure way of cleaning the barrel after a period of storage, no matter what I used to grease the barrel to protect against rust.