Get yourself a bottle
of Hoppes #9, plug the chamber, fill to the muzzle with Hoppes, and
leave over night.

I've found shooters choice works much better. Use it all the time (I'm a
gunsmith). Just scrub it with a very "wet" brush and leave it in for a day or
2. repeat if necessary. If it's jacket material, it will get it out. If
someone has shot cast bullets and leaded the barrel, it will need more
mechanical meas of removal (& JB Bore paste)
Greg Sefton