Mad Dog wouldn't be 'dedicated' AA, just a mech that can do it really well. Never said anything about strapping on shittonnes of autocannons

Also who said the arms can't move? Look in the picture in my first post, the guns are clearly aimed upwards. Hell don't even pay attention to that, check it yourself - when you aim up in a mech, your view actually aims farther up than the mech pitches... that's the arms moving up.