honestly, it's completly up to the group.
AA guns shoot where they're pointed and it's entirely up to design. the big quad mounts irl generally point up, but several large pieces (flak 88) work just as well pointed at the ground as they do the sky.
ask your group, since it was never officailly included in UWZ (or even 2cd as far as I recall) it is entirely up to your group.
-PFC joe