I have used TC Bore Butter for years and never had a problem with it. Mabey because I only used it after I cleaned my barrel. I'd run a patch of bore butter down the barrel then load it for the Hunt. If I shot, I cleaned the barrel with hot soappy water and reloaded again with the same procedure as above.
I also use a small amount of it on the breech plug threads and even coat the outside of the barrel with it, it seemed to protect it from rain and snow.
I guess if you use bore butter as a swab between shots and dont clean that often it would be a problem, but for me it never has been.
Last winter I changed to Montana X-Treme Bore Conditioner and their Gun Oil, not because I disliked Bore Butter but because I just wanted to try it out. So far no problems with it. Also when I used Bore Butter and after a Hot Water Clean my Barrels were always Shinny Clean with NO build up as others speak of. Whatever works for you is what you should use.
BP