If it's a percussion gun you can put the hammer down over a piece of leather on the nipple to close it off and then fill the barrel up with water. Let it stand upright for a few minutes and then pour it out, then you can repeat and cover the bore with your thumb and slosh it around and pour again. Do this a few times and it will remove most of the fouling, then you can use patches soaked in water and maybe a bit of ballistol as once the water evaporates the ballistol will remain behind as an oil. This is for real black powder, I don't know how to clean the substitutes. Depending how much you shot you will want to take the lock off eventually and clean the back side.
If it's a flintlock there will probably be fouling on the underside of the lock. You can also pull the barrel and use the bore mop technique to pull water in and out of the barrel with the flash hole submerged in a bucket of water.