If not you might be able to unscew the drum from the barrel
Do not attempt this on a CVA. They have a complex system that ties in the entire breech system. An amatuer trying to take this apart could ruin the gun. Removing the nipple (and later replacing with a new one) then flushing the breech with hydrogen peroxide followed with a soap and water bath will clean it up just fine. If there is a rusty bore, that is another challenged that might need lapping.
And, Bore Butter is not a necessity, it is just a gimmik product.