Unwrapping the inside of a barrel I'm trying to UV unwrap the inside of a barrel I made.

The problem is, when I select the inside of my barrel, and use the
Mesh > UV Unwrap... > Cylinder Projection, I get a weird unwrapped form. How can I solve this?
Cylinder Projection is based on the angle of the view.

Try switching to front (Numpad 1) or side view (Numpad 3), then unwrapping again: