There are actual mandrels out there for this, and are not expensive at all.
I bought all 3 sizes from Brownells, and have helped many a factory rifle shoot to its ability.
I even got a carbon buildup in a Hart barrel, that took many passes with JB to get it out.
I washed out the crown in the process but was able to bring it back with the mandrel, and it shot better than before.
http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=718110&t=11082005
