One thing that is usually available in the field is windshield washer fluid. It's either under the hood or in the toolbox. I never use anything that has vinegar or ammonia in it to clean my rifle. Windshield washer fluid will not harm the paint on your expensive vehicle,,,,,,,,soooooo, likely you can get the gunk out of the barrel without taking the finish off your rifle. Of course, I hunt with a traditional rifle that I built.
Often times I will just pour the barrel full of washer fluid and dump it a couple times, then swab it dry which removes about 98% of the BP salts. When I get time, then it gets scrubbed with hot water and soap.