a sci fi turret

I want to do something like a post-apocalyptic Scene with this turret and allot of ivy.
(I hope you know what I mean)
Anyway, the turret has some poblems with it’s lighting. At the cannon are some strange black parts and
I don’t know, why they appear…:spin:

Please tell me if you know how to solve it

Attachments


Sometimes when I get the strange black parts on a model I switch to edit mode, select all faces on the mesh then press W on the keyboard and click on Remove Doubles. I also click on Recalculate Normals, it’s on the tool bar under Shading/UVs. Most of the time it seems to solve the problem.

The normals are alright but I will try the “remove doubles”.
Thank you.