I have a 22-250 here that has a bore like a sewer pipe. A bit of rust and pitting on the exterior too. Not likely it ever had any corrosive ammo fired in it. Most likely zipped up in a case dirty and stored somewhere damp. I think I read somewhere the carbon in the bore can absorb moisture from the air. I make a habit of leaving cases unzipped a bit, and I've still had a couple of 'surprises' taking a wet rifle out of them. Never any damage fortunately.