One question regarding the plastic cases. I have read on numerous occasions that Black Powder burned too hot for use in plastic, that cases would be very short-lived. I have thus refrained from loading BP in plastic, so have no personal experience.
I have a W&C Scott 12 ga pinfire I have been wanting to make up some shells for, but for the work involved would want more than a couple of shots per each.
Where are you saying the Magtech brass is too thin? Is it for the purpose of swaging the rim or for holding the "Valve"? I could turn the rim so that would not be a problem. If there is not enough Meat to provide strength enough to hold the cap adapter that would another matter.
I have a box of 25 of the older Magtech 12 gauge brass which used the Berdan primers which would be a good use for them if I could work out a suitable conversion for them.
