Would suggest you might try a different J-B product, one called Bore-Shine. It IS a form of very light lapping, using a rod, patch and a product about the consistency of toothpaste.
Would be very leery of using customary lapping abrasives- too much. You can clean the fouling, but until you smooth the bore it will continue to build up. Happily, they still shoot pretty well even with a pitted bore. But overdo it, and you will have an oversized bore.
Re: hang fires- had some Pakistani .303 that was horrible (The CIA markings had nothing to do with the American Central Intelligence Agency!) That stuff had been poorly stored, and the cordite seemed to smolder for a half second before deciding to pop. I scored a quantity of Greek .303 I have been happily shooting since, but am now down to about 1000 rounds, so I may have to add some .303 dies to my reloading bench.