The currently sold Mag-Sparks use 209 shotgun primers. Usually they lift out, but can easily be removed with a small knife if stuck. The primers that sometimes stick for me are CCI Magnum. A spare cap doesn't cost that much.
I have mag-sparks on several rifles. They work fine, but sometimes the spent primer gets stuck and you have to pry them out with a pocket knife or leatherman tool. Also, the cap is easy to loose, so be careful. They do sell spare caps.