Was the "range drum" an instrument only present for artillery pieces firing in an indirect role? Would one be used in firing an anti-tank gun normally? I am guessing the former rather than the latter, so a 17-pounder with a range drum (amongst other things) would be able to fire HE indirectly.
Not strictly. Some tanks also had range drums for direct fire. They used a calibrated cam to offset the sight from the bore axis so that when the crosshair was brought on to the target the gun would be at the correct elevation above the line of sight. Well, that's how I understand it. A similar system might have been used on anti-tank guns, or they could be doing something else entirely...
The range drum was usually part of a "sight gear", which was an elevation offsetting mechanism. These were generally intended for indirect fire, but could be used for direct fire from "turret down" positions, i.e. with the entire tank hidden. The range drum on the AFV Sight Gear for the 17 pounder had two scales - one for 1800 feet per second (HE fire), and one for 2950 fps (APCBC).
Thanks Don Juan! The detail about the different shell velocities raises an interesting question... Was that the reduced charge HE? Only that wasn't available when many 17 pounder guns were manufactured. And did it have a different muzzle velocity? I think I'm going to have to get Dick Taylor's book on ammunition.
The AFV Sight Gear No.2 Mk.I for the 17 pounder is shown below. This was fitted to the Centurion Mk.I, and was probably also the sight gear used in the Sherman Firefly etc., although I can't confirm this. Not shown is the clinometer (a small spirit level) that fits on the clinometer bracket. This bracket is also known as the rocking bar, and this type of sight gear is often known as a rocking bar sight. My understanding of how it works is that the gun is aimed directly at the target to give a line of sight, and the rocking bar is set so that the clinometer is level. The distance is then set on the range drum via the handle, and as the range drum rotates, so does the rocking bar and clinometer. Once these have been set, the elevating gear on the gun is turned and the gun elevated. As the gun elevates, the rocking bar rotates back to its original position, and when the bubble in the clinometer is once again level, the correct elevation has been achieved.
Yes, I think the range drum for the 75mm was the No.1, and the one for the 77mm was the No.4. I'm not an expert on this, though, because this is a rarefied subject even for me.
The principles of these sights are described here. SIGHTS AND LAYING Here is a example for the multi charge 25 Pounder Gun Howitzer. This is a more complicated but follows similar principles.
