Nik,
The less taper in the wall thickness the better. It will work on just about any gun you can get an accurate wall thickness measurement on. However, nearly all the sxs shotguns I've seen have lots of taper on the outside of the chamber. This increases the likelyhood of a measurement error by the fact of where it's placed on the chanber and the span of the gage itself over a taper.

I looked for quite a little while to find a short chambered singleshot. But, it's the simplest way to go. I bought a 12g and a 16g as well.

I've thought of ordering a barrel blank and making my own bench pressure gun for this purpose, but I'm not trying for absolutes here nor do I test much. I worked up some loads a couple years ago and haven't taken it out since. This is the kind of thing a group of guys or a club could really benefit from. I bought it specifically to work up some loads for my damascus guns. I found my 1 oz load with components and pressures I was satisfied with and stopped. I'm thinking about working up a 1 1/8 oz low press load someday, for those phez trips to SD.