Shoot it !!
I have a 1941 German M98k with a badly corroded and rusted barrel, (it wasn't cared for very well, before me). I shoots fantastic, I have put 3 shots into 1" and 5 into 2 @ 100 with iron sights (Mojo rear).
When I clean the barrel; the patches (after brushing) keep coming out dirty grey, I can run 20 patches and they all come out the same, I have even cleaned it with a hot barrel, same thing.
I gave up getting upset about it, it shoots so good, I just clean it and accept it.
Maybe the rifling is tighter cause of the corrosion! ha ha !