Take a look at
this. Please take a look at the spelling of the word "mou
nted" in the thread title. Taking the time to catch small details pays off, in thread-starting and map-compiling (or so I've heard). As to changing the damage effect, ==Troy== maybe able to help you with that, too.
By the way, what do the Aliens get in your map to balance this huge Human advantage?
Also, if you can't make it aim-able, just fire-able, build in a choke point in the map construction.
Note that the above link is to a turret that shoots Humans as well as Aliens, not exactly what you are looking for. But if he could do that, ==Troy== might be interested in messing around with turrets some more.
P.S. During the change from phpBB to SMF, all the first characters got clipped off of sigs. Yours would look a lot better if you took a minute to add back the missing "[".