I would not bother lengthening the chambers. It's easy enough to work up light 3/4 oz target loads using standard American 2 3/4" hulls. Pressures will be well below that of a gun that has passed French proof. If you're planning on hunting with the gun, I'd just buy 2 1/2" RST's for that, or else something like the Gamebore British 2 1/2" loads.