Hi:

I may be a bit provincial, but I have been using a pencil eraser in a decapped shotgun shell for my snap caps for years.

Just decap the shell, push a pencil eraser into the primer hole and sand the eraser until it is flush with the head of the shell.

These appear to work well for me and the price is right!

I always lube the chamber and snap cap to prevent galvanic action!

Stay well my friends,

Franchi