Look at it this way, A barrel starts out with nice sharp areas of the corners of the rifling . Along the way you build a big fire in it a few thousand times and it burns the corners off. Now take a barrel that to break in you put an abrasive on a patch and run it in and out. The result is that you take the corners off the rifling so that all that fire which would have started with sharp rifling is now starting with rifling that is thousands of rounds old. Which means that a lot of the life is gone. A lap always cuts more on each end where the compound reverses direction as it starts back through the barrel which means that it is enlarging the bore at each ends of the barrel. And last picture a patch riding along the barrel with abrasive on it. It is removing material at a given rate. It comes to a place where there is copper fouling and it rides over it cutting the same amount that it was cutting before it came to the copper. You continue until all the fouling is gone and what have you done? You have put the came contour in the barrel steel that was in it when it was metal fouled. It would not be as bad if it were used on a lead lap but I ask why would you want to abuse the barrel when you can accomplish the same thing without the bad side effects. There is Sweats, Otters foul out or just a good daily cleaning with a good bore cleaner till the fouling is gone. To top this off I will relate a true happening. I built a bench rest rifle for a customer and as usual I fired 5 groups of 5 shots and calculated the aggregate. It was good enough to see that the rifle was capable of winning the Nationals so I shipped it. I got a call from the new owner saying how happy he was with it the way it shot. About 4 weeks later the rifle showed up with a note saying it wouldn't shoot. Sure enough when I tested it it was shooting groups three times the size if the ones I had shot before I shipped it. When I bore scoped it the barrel looked like a mirror and the rifling wasn't square it was half round. From that time on I put a flyer in each gun saying if any abrasive was use in it voided the Warrantee.
Now I am not trying to stop you from doing what you want but just inform you what is happening when you use JB. Brass brushes are softer than barrel steel and does no harm. S/S brushes are harder than barrel steel is definetly a no no. Nylon may surprise you to know is very abrasive If you doubt this look at the carbide eye on yout fishing rod where nylon line has worn groves into it.