I think the Repulsar cannon is only supposed to track flying Tech. So as a weapon it's a total flop. Especially as you canplace it in one of four directions, all with the thing pointing up. and it's probably not going to be fixed until we get a reliable flight AI.
I haven't really messed with the AA gun since 1.0.2, but it definitely worked back then. I think the devs had enough of people tilting it to point forward (the cab rotation trick, witch still works btw), so they reworked the code and now the gun doesn't even fire, when it's in the "wrong" orientation. Unfortunately, they also introduced this bug, turning the gun from kind of useless to perfectly useless.
Well, It was the only large turret that would not only punch through shields, had a fast reload & retarget speed and more than enough range.
They simply forgot the player's mind set to reorient things so that they could be aimed properly despite intentions. So they nerfed the thing.
As it is now there are better weapons for anti air, at a smaller size. they realy should either repurpose the model, or remove it entirely.