The Royal Navy's .50 cal Vickers Mark III was generally found to be ineffective as an AA gun. The main failings of the gun was its short range, low rate of fire, and the inability of the shell to do critical damage to a target aircraft. The .50 had an effective range of just 800 yards, compared to 1000 yards for the Oerlikon gun, 1200-1700 yards (depending on shell type) for the 40mm Pom-Pom and up to 3000 yards for the Bofors gun. For AA defence, this meant that the .50 cal could frequently not destroy an attacking aircraft before said aircraft had carried out its attack. The gun had no inbuilt guidance, and was not connected to the ship's AA directors. As a result, what was called 'eyeshooting' had to be used. The guns were fitted with a sight consisting of two or three concentric rings, each representing 100 knots of the target aircraft's effective airspeed. By pointing the gun such that the aircraft was behind the appropriate ring, and flying towards the centre of the sight, the appropriate deflection was given. This was only an approximate method, and corrections had to be made by following the line of tracers. This was most effective with a gun with a high rate of fire, unlike the Vickers gun (effective rate of fire 150 rpm), or the Pom-Pom (effective rate of fire 96-115 rpm). The shell fired by the Vickers gun was a semi-armour piercing round capable of piercing up to 15mm of armour at 100 yards. However, it was a solid-steel round with no explosive charge, unlike those fired by the other guns. This meant that, to score a kill, hits had to be achieved on the target's engine, cockpit or other such vital points. Pre-war calculations suggested that a ten-second burst from a quad mount for the .50 Vickers would put 40 rounds onto an aircraft target. However, this assumed perfect accuracy by the crew, a highly unlikely occurrence. Realistically, accuracy was 25% of this or less. When so few rounds were put onto the target, there was a low probability of hitting the important targets. With a gun firing an explosive round, those few hits were much more likely to do vital damage.