How rusty are we talking?? Light surface rust? Or practically rusted closed? (I've seen one that bad...an acquaintance thought stainless doesn't rust and his dehumidifier in his safe would be fine...no need to clean..).
Make/model? Condition of the rest of the gun?
If the bore is not salvageable and the est of the gun is in good shape and worthwhile there are options of having it bored out or replace the barrel...
I'd start by soaking it down with a good penetrating oil for a while...let it soak and work...then get at it with normal brushes and patches to see just how bad it is...
I have a Rusty black powder barrel, or two or six, and mine all shoot just fine!
Hard to load though right?
Lol, nope, min aren't. I was just making a play on words with my moniker.
But yes, a corroded barrel is very hard to load, especially with a patch and ball. The patch catches on the rust, and gets ripped or snagged. Those rips or snags will badly affect accuracy because of burn through or blow-by.
If you can even get the PRB down against the powder.
“There is Truth in the Old Saying, That if you make yourself a Sheep, the Wolves will eat you.”
Benjamin Franklin, 1773
I've never tried it but an old timer has told me that he has soaked many a rusted barrel in cheap cola. He used a small piece of gutter and would soak them for a day or so then wipe them down and clean as usual. Again only second hand info.
If an inline is just like a flintlock why don't you get a flintlock so you can hunt more seasons?