I just drop a set of shotgun barrels or a rifle's barreled action into a PVC tube, fill it up with boiling water, cap it and let stand for 10-15 minutes. Simply works.

I plug both ends of barrels with tight fitting tapered wooden plugs, but expansion of heated air inside the barrels tends to force them out, letting water in. A swab pushed through afterwards takes care of that. I thought of dispensing with the barrel plugs altogether, but decided they have a use for while the barrel is rusting in the sweat box, so I put them in.