1. the turret is operated by the driver, a passenger gun by a passenger (like the name says). So in your case you have to use turret/barrel
2. in your config for the vehicle you have to add/edit this 2 lines
Driver 0 38 0 -360 360 -10 20 <- Position of the driver, to read as posx/posy/posz/turret limit left/ turret limit right/turret limit down/turret limit up
RotatedDriverOffset -5 0 -6 <- offset of the driver from the pos in the line above in px
3. All parts of barrels and turrets have the same position. The position of the turret is normally 0/turret y position/0, the barrels position is the normally the potion where the barrel is rotation around.
All parts are moved the with the offsets to their position (offsets can be set left under parts position and size)
4.you have to build the missile as part of the model. it will be displayed always as part of the barrel in your case.
check the attachment i put to this post. its y tank but you can see turret and barrel