I have a Kreighoff .22 WRM insert barrel that I got for a Savage/Valmet O/U combination gun. It shoots very accurately and has an integral extractor that works with the original shotgun one. I have it set for the open sights and use the scope for the centerfire barrel.

I got mine used when a friend bought a modern drilling at an estate sale and didn't want the insert. Not sure they are available any more and would be pricey if they were.