if the original poster has access to a metal lathe, I would suggest pulling the breach plug and then opening the bore by pulling a series of spade bit (D bit) reamers down the bore and stopping short of the threads for the breach plug. I would also consider turning the front half of the barrel into a tapered round barrel to get rid of weight as well as remove the under rib and I guess soft solder so ram rod pipes to the bottom of the barrel. 7 pieces of buckshot, if 00 buckshot plus a ball seems like a pretty heavy load and i would expect the gun to shoot quite a bit low. How many buckshot balls to use depends on whether the game regs say single projectile prohibited or say X number of 00 buckshot required
cheers mooncoon