 The main equipment of the Heavy Anti Aircraft artillery was the 3.7 inch anti-aircraft gun which came in two versions, static and mobile.The main difference between the two types of gun was that the mobile has the outrigger legs and the static was bolted to the ground and had a large arm atop to counter the weight of the barrel being slightly further forward of the point of balance.It is believed that approximately 50 mobile units are known to survive in Australia.