I did take mine apart and it was bone dry when I bought it. I took Flitz to the slide rails and oiled them. I also used flitz to polish bolt group n chamber. I was trying to smooth it out but to be brutally honest, it didn't help much that I could tell - smoothness wise. PIA to reassemble 20 ga.
The roughness of the interior was something you could feel w your fingers. I asked my gunsmith if I should try to stone it but he advised against it. He confirmed 20s were PIA to reassemble which made me feel a wee bit better...
He also told me to oil it and then kick back n watch TV and cycle it like a teenager w first gun. This will wear it in & that's about as good as it will get & safer than stoning.
Note: Not classic Wingmaster smooth but it will cycle anything I feed it. Have no issue w calling it a SD gun. Was actually looking for an 18" tube for SD w sights or rifled slug barrel but budget limited. Is it luck of the draw ? Cleaning ? Flitz? Cycling ? Or all of the above. No idea - it just my .02