Well, if you don't get liners, you are going to have to have the rifling cut with a broach. And since the bore is already nominal 20 gauge, you are going to need to use an oversized or hollow based Minnie Ball type projectile to get good rifling engagement. Assuming you have a typical chamber barrel arrangement, it should be possible, and the Minnie ball did work. It was undersized to allow muzzle loading and expanded to engage the rifling upon firing. I would think someone who re-cuts rifling might be able to help you out. Even though it is thick, I believe I would get a material Spectro anaysis to get a handle on the type of steel you are dealing with. Shouldn't be too expensive in that you could take it to them rather than have to pay for a field analysis. You should have access to this technology in Germany. Having a field test runs about $1,000 over here, and they can tell the difference between 304 and 316 stainless, so it's pretty accurate.
Good luck.
