You need to know the load that is in the existing shells but you'll probably never find that info. Second you would need to know the load for the one you are doing. For this you would need a loading manual. The loads are almost guaranteed to not be the same and probably not even close. With the wrong loads you risk literally blowing your gun up along with your hands/face. Best advice I would give is to shoot the existing rounds and then reload them after you research the load you want. They can be loaded using some very basic hand tools and knowledge. I use tools like this to load black powder shotgun shells.