There are quite a few posts on the subject of cleaning ML guns so we probably don't need another. But I'm going to post one anyway as it may be of interest to fellow Black Powder purists out there.
I've noticed there are lots of shooters who use petroleum based products in their BP guns, WD40 being commonly used for drying out metal parts after cleaning. Maybe it is because of the fact that the humidity was so high when I lived on the east coast, but my experience with WD40 was uniformly bad. Every time I used it by the following day there would be surface rust inside and out, even when lubed with Break Free or some other rust preventive. Ouch.
After a few times doing that, a friend recommended cleaning with straight hot water only, followed by generous amounts of Bore Butter. Still, there was the problem of residual water in the barrel and what to do about it until one day when dear ol' Dad suggested using compressed air to blow the residual water out of the barrel. Ever since then, my cleaning regimen is to remove the nipple (and clean it separately), remove the barrel, and scrub the BP residue off all external metal parts with a bronze brush. While all that is going on I heat the water and charge up the air compressor. I use water as nearly boiling as possible, submerge the breech, and pump the barrel repeatedly with a tight fitting patch. I repeat the process after changing the water two or three times until water coming out of the barrel is completely clear. Next is swabbing the barrel with three or four dry patches to get most of the water out, then blow out the remaining water with compressed air. Something that helps is to wrap the air nozzle with electrical tape to form a seal with the muzzle and prevent damage to the crown. Since the barrel is still very warm, any remaining water dries in about thirty seconds or so. After that I bathe the barrel inside and out with Bore Butter and let it cool completely before re-assembling the gun. Using this method, I have had no problems with rust and once cleaned the gun can be put away with no worries.
The variation for flint guns is to plug the flash hole with a plug carved out of hardwood and use a measuring cup to pour the water into the barrel, the rest of the process being pretty much the same. Keep the vent hole side of the gun facing down so any water leaking from the vent doesn't run inside the stock.
Haven't bought any WD40 in a very long time.