American guns typically had thicker barrel walls and were honed less. More American guns were of a lower value and did/do not justify the cost of sleeving. Sleeving was much less known and available here as compared to there.
Fish - you speak of a project that is a bit more complicated that just sleving. How about doing this with rifled choke tubes and one of the temporary type of sight systems. If you really like it, you can go for a gauge rifle setup. However, I'd investigate a whole new set of barrels from Belgium or Spain prior to cutting the existing barrels. I've a sneakin' suspician that you are envisioning a "on the cheap" O/U rifle and I'm doubtful that it will turn out that way. No doubt you can build it - it is the cheap part that may not work out.
