I've never seen a 39 Marlin that didn't shoot the lights out. If one didn't, I'd start with a good barrel cleaning and then dive into as many brands of ammo I could find samples for. I'd rather live with the best results I found from the samples than EVER firelap or even JB the bore. I've JB'd a lot of centerfires, but I think the risk is a bit high in a 39, even with the receiver apart.

Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy, its inherent virtue is the equal sharing of misery.
--Winston Churchill