there is a PPK conversion on Simpson Limited's webpage that fires this stuff now, its not terribly expensive.Somebody on RimfireCentral had a little 4mm Schutzen rifle for sale (might still be there). If I’d had the $1800 he was asking for it, I’d have bought it! Such neat little rifles!
Edit: Still there!
I am not sure what you would call these as far as ignition, its not a rimfire, centerfire, or pinfire... since its just a primer cap with a bullet.