#1). Obviously, you can "scrub" it out with brass bore brush(es)... many and alot of elbow grease. Seems "right" for the person who overloaded the "plinker" bullets and made the problem.
#2). Easiest, and HARD today, fill the barrel with mercury. Hard because getting mercury is no longer easy. Mercury amalgamates with the lead and pours right out... Overnight would do it, IF you can come up with liquid mercury metal... EPA put tons of restrictions on it and you don't pop down to the hardware store for some anymore...
#3). I would look real hard at the black powder bore cleaners because they shoot pure lead bullets and might have something formulated to remove lead quick and a lot. Otherwise, the "carb cleaners" --OLD-- are also known to dissolve lead. It used to be in gasoline. From there, the same chemicals in different bottles, are sold as bore cleaners and enough of any one of them would do the job--see #1). Hoppe's #9 is lame. Shooter's Choice, many times faster. Same chemical is supposed to be in a GM carb cleaner. Lead dissolves in many, many organic solvents, so you could use Gin or vodka if you wished, but what a waste. Have a party. Get some people to help scrub... (ha, ha).. luck.