Watching a visual guide, such as this YouTube tutorial for a Sawed-Off Shotgun , can help with tricky assembly parts. Would you prefer: A simple 5-minute design ? A full tutorial for the working model that shoots?
Add a mechanism in the loading gate to eject the cartridges. Tips for Success shotgun
Create a slightly thicker roll for the body and attach the barrels to it. Watching a visual guide, such as this YouTube