>>96377567
>small hole with the tip of my scalpel
Thats your problem, use a pin to make the whole so its actually round. After that its just about starting small and working your way up. Start with a #74 and go to #64 for boltguns. Make sure to check your centering each step from different angles of lighting. Its really easy to end up thinking you are center because of the shadows but then you turn the model to the other side and its totally wrong. I normally do this by spinning the model upside down and to the sides to check, while holding the model on either side of my lamp. Takes about 5 min per barrel.