Applying a penetrating oil and letting it sit for awhile before applying the steel wool will help. Kroil, PB-Blaster, and Gunzilla all do a good job. I've seen a sectioned shotgun barrel that was rusted on the inside and that application of Gunzilla and patches alone had pretty effectively cleaned. All the penetrants do best after they've wet the steel for an hour or so.
Flitz, Maas, and Gunbrite all remove a certain amount of rust, but are not great rust removers. They do best if you can clean and boil the rusted part for fifteen minutes in distilled water, first. That turns a lot of the fine rust to black magnetite and it rubs off. What stays behind is then blue instead of red rust. The steel needs to be degreased before boiling.
