Jaz,
No offense intended, but we have mention 3 guns in this post and I was interested in Sarg's gun's chamber length and I also wanted to acknowledge to whom and what my responses were directed. That does not preclude answers from all members.

I think JBHunter is right about the charge and bullet weight on the barrel flats of the Julia Holland.

Sarg,
I love the engraving on your 12. What do the barrels weigh and what are their length? I think you are right, it probably is intended for a large charge, 8 drams+ maybe 10.