I have never heard of using any type of oil in a barrel to prevent leading, and agree that it would be dangerous to put oil in a barrel, but as to the question of using something in a muzzle break, I have heard that for supressors that normally require packing with grease, you can spray WD-40 in the suppressor instead. It only works for a few rounds, but is a lot easier than packing the grease in. Your question is probably about keeping someting clean rather than a question about using a spray lubricant in a functional way, in which case please ignore this.