Cleaning BP out of smooth barrels is a snap once you get used to the idea that plain water sloshed all over them won't hurt a thing!
Dismount the barrels. Pour a pint or so of cold water through each bore. Use a few patches to wipe out the residue and dry things out. Dry the barrels inside and out. Smear a bit of your favorite oil inside and out. You're done. No evil chemicals and no rust from the water, honest! Now that I'm used to water on metal I much prefer cleaning BP to cleaning smokeless.
If the bores are pitted it's a bit more work as the BP fouling likes to hide in the pits. A few more moist patches and a bit of bristle brushing will help. It's a good idea to check on pitted barrels the next day, too; maybe swab and brush them a second time. Unpitted barrels really do make BP cleanup much easier.
If you think you might want to use plastic wads with BP expect a disgusting mess of melted strips of goo in the barrels. Don't do it.
