NT,
I have a 12 bore Holland 7 dram gun made in 1882.



The gun weighs 13lbs and the barrels abit over 9lbs. I use brass cases but have used plastic in the past. I have a period mold which casts both ball and a short conical in the .740 range. My gun is proofed as a 12 and has a groove diameter of .736. I too would be interested in seeing photos of your Holland. Here is a reprint of Frank Findlow's chart from his excellent article in DGJ on 12bore rifles.