My comment on not leaving it in for longer than 15 min is based simply what it says on the bottle. I tend to follow the instructions. If it says not to leave it in longer than 15 min, I will not.
I am no metallurgist or chemist, so I do not know what will and will not harm a barrel on the chemical side.
Still waiting for the KG12 I ordered to come in. If it does not work, I will plug the bore and fill it with the sweets.