Hello i have been worrking on one tank but the problem is in that i cant set the pivot to be in the turrets body not in the barrel.Even if i press Pivot it doesnt places it in right place.
You could make it a child of another GO and use the parent GO as your pivot. Or just center the object in your 3D app before exporting to Unity.
OK thanks
The pivot is set in your modeling app (Blender, Cinema etc.). The solution Frank mentioned - parenting another GameObject to it as pivot - is a valid way of doing this but be aware that you might run into problems which object actually is rotated. By default Unity will probably select your child object and not the GameObject you used as a parent when clicked in the editor. So if you’re not careful you might end up rotating both objects separately and create some frustrating errors difficult to track. Believe me I know that … I sometimes use that method for different purposes in Cinema4D.
The better and cleaner way is to set your pivot in your 3D app. Just HOW to do this - depends on the Program you’re using. I can help with cinema 4D and a little with Blender. All other applications I can’t tell you without having the application open in front of me. 
Im using 3dsmax.
In 3ds max, select your object, on the side panel tab, select hierarchy - select affect pivot, hit center to object. You can actually put your pivot wherever its useful, so sometimes for doors for instance, you can set your pivot where the hinge should be, to get your doors to swing properly on rotate