You can actually scrape, too. I have.
I have made some reamers, all from very thick and large files. It's quite tricky, as they have to be made to rather close tolerances, but not impossible. The tools I use are an angle grinder with a cutting wheel, as well as a grinding wheel, a stationary belt sander and a grindstone. What you need to aim at is recreating a curvature as close as you can to the innner conical shape of the bore. That is the shape of the reamer will be something like this: /_____\ rather than \_____\ or |______\ . (Add a connecting line on th top as well.) The reamer will have a straight grind on the cutting side, but a curved one on the other, so it sort of rides on the inner curvature of the bore. Of course drilling a progressively telescoping bore will ease the reaming. All these reamers work scraping, and if they are kept sharp and smooth, produce smooth bores.