the turret firing point limit is 4 points for missile turrets and 3 ONLY for laser ones. just look in the weapons editor in fred. the window only will fit 3 laser.
i have tried upping that limit using the source, but it crashed no matter what i tried.

(and i DID remember to change that window and all the related code. i think i'll leave it untill i actually learn to program.....apparently there is a lot of code that assumes there is only 3 tfps.)
anyway, multi-part turrets are quite easy to do. as Bobboau said:
-you have the turret arm geometry and base in their correct positions.
(i would suggest compiling the parts in a separate cob/scn for simplicity and then add the finished single object to the objects library [ts5 feature-not sure of others])
-gule a local light to the arms as a sibling (don't think this is nesacarry, but it's never caused me problems) and set the axis of the new object group to the bottom of the arms-or wherever the "hinge" would be.
-glue another local light to the base as a sibling and set that object groups axis to the center of the geometry. or the point that the turret is attached to the hull and will rotate around.
-glue the arms object group to the base group as a
child and name the object groups accordingly.
hope this helps, but someone correct me if i'm wrong

([SIZE=0.3]just not tooo painfully plz

)[/SIZE]