Guess you were posting at the same time I was.

I expect you need to do more than run the boiling water through. That rifle might be corroded to the point of being dangerous. Get her broke down completely and cleaned as best you possibly can, then do a careful visual inspection at the very least.

Store that bore butter somewhere out of sight, put some good gun oil in that bore and some good anti-seize grease on the plug and nipple threads. Patch it out with alcohol patches then dry patches before loading it up.

You might consider some fresh powder as those old pellets are probably bad.