Yes you can use the gun on drive-able BTRs, but the system is a bit clunky. From memory, you aim the turret with the mouse, wait for the "target locked" signal and then fire using the [TAB] key. The turret only turns part way, so I found it easier to reverse away from enemies so they follow you.
EDIT: thinking back, the turrets may auto-aim when you are near an enemy. I do seem to remember it never pointing where I wanted it to!