Your turret is either too detailed, or the details are not shaped correctly. When I compiled the map, there were many 'coplanar' errors. You'll have to rebuild certain pieces of the turret. In your compile log, notice the "Entity 4, brush 17" entries...it lists many brush numbers. Each of these brushes will need to be rebuilt or deleted; they are causing the problem. Find them in worldcraft by using the Go-to-brush function (Ctrl+shift+G). It looks as though you have carved some of your brushes to get them to look like they do. Try making several separate brushes placed together to acheive the same shape. Simplify the shapes as much as you can. The double barrel will cause you trouble later. When you want to shoot the tank, you won't be able to make bullets/rockets come out of both barrels.