Actually, you can use a combinations of expressions and parenting to achieve the effect - no aim constraints needed.

Although I've found that the system doesn't point at it exactly - probably due to rounding errors (it gets noticeably better the farther away the target gets). But its pretty close regardless.

Setup - basically, just parent your gun in your turret to your turret base (or to whatever controls you have). Then, you'll need some variant of the following equations;
for the base:
base.rotateY = atan2d((target.translateX),(target.translateZ));
- this basically points the base at what would be the target's position seen from above.-
for the gun:
gun.rotateZ = atan2d(target.translateY, sqrt(target.translateX * target.translateX + target.translateZ * target.translateZ));
-this points the gun at the target vertically, and assumes that the gun is already pointing at it horizontally.-

the second equation is a little bit messy, and that's where most of the divergence is seen (most likely due to rounding errors). You should be fine with anything over 20 units away, though.

Actually, by bringing this up, you may have helped me come up with a way to solve an old problem of mine. Hope this helps!