mauser06

If you are shooting real black powder the recommendation has always been do not use a petroleum based product.

If your bores are dry and remain dry BB will not cause a problem. If you are shooting real BP you might even consider Wonder Lube 1000 - when it drys it seals - but again make sure the bore is dry. It can easily be removed with a simple wet patch. No matter which product you use if it is not an oil and you are in a humid climate - check the bore on a regular basis - even you use oil and live in a humid climate check the bore.

Bore butter left in the barrel will cause leaching if the barrel is exposed to heating and cooling on a regular basis. With heating and cooling the BB will leach into the pores and force what ever else is there out, including powder residue.

If you are going to store the barrel for a long period of time in a semi-humid to humid climate - oil it - actually grease it. When you decide to use it again clean it with boiling water - the grease is removed and you are back to clean bore.