Firing a bullet through a rusty bore can destroy the gun, depending on how much rust is there.
This is essentially firing a bullet through an obstructed bore.
Too much rust and you get anything from a bulge, ring, or blown barrel. Even with lighter rust pressure soars.
Here's a professional way...
Flood the bore with Kroil and let soak a day or so to soften the rust.
Use a new bore brush to make 5 to 10 passes through the bore, applying Kroil every few passes.
If the rust is heavy, use a stainless steel bore brush. This is the only time to ever use a stainless brush, they can damage the bore.
Use Kroil and JB Bore Paste to do the final clean.
Once done the bore will be in as good of condition as it's going to be.
Using abrasive coated bullets will mostly affect the throat, not the rest of the bore.
Don't make the mistake some people do, which is to spin a brush with a drill in the bore.
This is a valid technique for SMOOTH BORE guns, but it will ruin a rifled bore quickly.
Once the rust is out what you have is what you get, which will be a frosty looking bore with fine pitting.
"Removing" the pitting is not possible since that would require significant metal removal and a badly oversized bore.