tinker
your gun have an unusual small barrel dia, if the word "unsual " exist for pinfire guns. the only standard we know is the old RWS bullet catalog. the 16 bore "catshead" bullet have a .711 dia., the upper "stopring" have .716 dia. .
the 20 bore have a .657 / .659 " bullet.
I think you can load any blackpowderload you can get in you case,it will be safe.
The case length your gun was chambered is 40mm / 1,57". what you have measured with your old cases is only a "home"cut. pinfire case for bullet cartridges are 33, 40, 50, or 65mm long. bigger ones like 10 or 8 bore can be found with longer cases. the fact that your gun have such a small bullet diameter shows it was build for paper cases. RWS had have 2 different bullets for brass and for paper cases. the paper case have a thicker wall and this help to fit your small roundball. I am working now only with Magtech shotgun brass cases, have made pinfire cases from paper and plastik center fire shells, also from copper tube with steel base. lot of work but only the magtech brass is it worth. I have an old box with 100 unloaded shells also old reloading tools but never shot an old case. collectors dont like to destroy collector stuff.
I believe you have a lathe, rigth?
working with magtech brass will be the best, you have work with any basic case. the magtech brass will survive all other many times.
I am a maker of rare cases, if you are interested.
