We frequently are asked by new members what is causing their guns to FAIL TO FIRE. It often comes down to using petroleum based oils, and failing to flush them out with alcohol before loading the gun. The temperatures created in the breech of these guns simply is NOT HOT ENOUGH to burn completely these oils. You get, as a result, a combination of Hydro-carbons from the unburned oil, together with the carbon, graphite, sulfur, and potassium Nitrate residues as "GUNK" in the tiniest of places.
These generally are the small orifice in the nipple of percussion guns, and the flash channels of those same guns, and some flintlocks. To clean them, you end up using a wire( Nipple wire) to clean out the orifice in the nipple, and then use the same wire to clean out the narrow flash channel to the powder. As Necchi says, its a royal PITA to do, and a lot of unneeded headaches in the field. Flushing the gun with alcohol( or acetone), to dissolved the grease and oils before going shooting or hunting is the ONLY sane thing to do. Other than, that is, using a Non-petroleum( synthetic, or vegetable based) oil to lube that bore.
Since this is a traditional forum, we tend to recommend the use of vegetable based oils for use inside the bore of any gun, and stay away from the synthetics.
However, If you are going hunting in very low sub-zero temperatures in Alaska( -20 degree f. and lower), or the arctic- I have no idea what there would be to hunt in such cold conditions-- then the synthetics, made to lube Aircraft engines, is the only way to go.