I use it when cleaning a copper jacket fouled barrel prior to testing cast lead bullet loads. It works fine but IT IS AN ABRASIVE...a little goes a long way.
In use...WATCH THE MUZZLE. If you allow the patch or swab coated with J-B to exit the muzzle, you are in effect "sanding" the edge of the rifling there...and that my friend is the #1 cause of barrel wear. Military barrels (the M1, M1 Carbine and any others that were cleaned from the muzzle end) were ruined by the use of steel jointed cleaning rods scraping across the crown. Remember that the rod is coated with J-B as well as the patch or swab and can and will remove metal where ever it touches...the crown or the throat for that matter. Being anal over the muzzle end of any rifled barrel is a good thing.
Guide the rod with your fingers so it doesn't touch the crown, or better yet use a bore guide. You can make your own from a .223 or 25-20 case with the head cut off, slipped over your .22 cal. or .30 cal. dia. Dewey cleaning rod. The bottle neck end of the case slips inside the bore, effectively preventing the rod from touching the muzzle crown. This system works for rifle or pistol, just vary the size of the shell you cut off or drill out to fit inside the muzzle.
When you're done...thoroughly clean out the residue with Hoppe's, Ed's Red or what ever your using for normal cleaning...and don't forget the rod you used. I like Dewey's coated rods and wipe them down after each use.