I am an amonia fan too, though I use 10% solution in quart bottles from Ace hardware for $2.50. The weaker 3% cleaning solutions aren't strong enough to do the job while the more concentrated solutions are too caustic to my nose hairs
and add little benefit beyond the 10%. I never let it set in the barrel loner than 15 minutes and follow it with dry patches, then oil. As long as you don't let it set overnight corrosion will not be a problem. Try wipeout and or the K product line then follow up with 10% ammonia, then reverse the order with another fouled barrel and compare...$2.50 worth of ammonia will last years and outperforms the heavily marketed cleaners for copper.