They work well in revolvers, or at least they work well in my Rossi 515s and S&W 48s. They also work well in Savage 24 combination guns, but they won't feed though any magazine that I've seen and the rim configuration may bend the extractors on some bolt action rifles. I know this because I forced one into a Marlin 882SSV and had to knock it out with a cleaning rod.
You could try making them by sanding most of the rim off a fired 22 Magnum case and then cut a couple slots from the mouth back to about 1/2 way to the rim. All they do is keep the smaller diameter 22 LR cartridge centered in the chamber.
