Gas operated semi autos may need some fine tuning to get them to cycle with a shorter barrel. Inertia guns tend to work pretty well without any tinkering.

Over/Unders also have certain factors to consider. If the barrels aren't properly cut down, you can have issues with the mid-rib separating that holds the barrels together. You may need to do some solder work after cutting. There's also the issue of barrel regulation. Over/Under barrels are regulated for a specific POI at a specific distance and then soldered together. If you cut it down, you may find the POI for each barrel is wildly different.

Overall, it's probably easier to make a semi auto SBS run than to get an O/U SBS that actually shoots well.