these are the good snap caps we used to make and I found one last pair. 12 gauge. nickle/chrome plated. ALL BRASS with the spring loaded primer pocket. The insert is a lead/steel alloy that will not damage the firing pin.
One is brand new
One is new with slight damage to the rim but fits and works ok,
45 dollars for both including the shipping.
You won't find a better snap cap around. These are the "chanpion brand" and the fellow that made them passed away about 5 years ago. They were state of the art.