Onionskin.....minimum 25% rag content, 9#.....002" thick after it dries. Doesn't hardly get any better.
Dress pattern paper is a winner, usually at .0015".
I realize most of you are doing a segway from cast bullet shooting. Put that knowledge on the back burner, mebbe read Matthews' book and follow his guidance. Proper geometry and materials will give you precision and velocity equivalent to jacketed bullets.