Good base so far. Triple barrel action is always handy
Don't forget to include the ram rod (maybe in the center of the barrels?)
I think the proportions are good except for maybe the trigger is a bit small and the stock (butt) could use a bit more length and girth at the end. Remember, the stock is supposed to be (somewhat) comfortable when you hold it against the shoulder. Shape it accordingly, and it will look much better.