Backlash in Enfield target sights
Backlash is present in any screw and nut assembly and any form of parallel engaging slides. Both these are present in target rifle sights.
Manufacturing tolerances cause parts (even hand selected parts) to have small amounts of clearance. There can never be a perfect match of the screw to the nut as it would not run freely when any distortion or contaminant was present. This clearance means a nut can move on the screw when no load is present. You can feel it on normal commercial screws.
Most sights adjust in very small movements, between 0.008" and 0.002". These tolerances increase with normal wear, and most of the sights referred to on this forum were made between 1908 and 1960 - they are very old and worn badly in many cases.
The effect of backlash on a sight is that the sight setting can move if the design does not lock it. The early Parker Hale range of sights locks the elevation setting with a small knurled hand screw for this reason, but the careful shooter will check the actual vernier reading after is is locked if wear is present. Having to check the setting slows your shooting down and the wind might change again while you are doing it.
The most used adjustment is the wind arm and this wears fastest. Both the slop of the eyepiece mount in the sight frame and windage screw wear are affected. Few .303 sights allowed for compensation for this in their design. Some like the PH 5 C can be rebuilt when wear is too much with shims or shim springs in the windarm.
The end effect of all the slop present is you move say 2 minutes left and engage the nut onto the thread on one side. The wind changes so you go back two minutes, but the shot still falls left. Why? the sight has at least 2 minutes backlash and the adjustment has only changed the engagement of the nut from one side of the thread to the other. This sight itself hasn't moved at all. I have dial gauge checked some sights with backlash of 4 minutes or more. A sight with this much wear is unusable. Any wear more than a part of one "click" (or more than 1/4 minute) makes the sight difficult to use.
The first design to allow for backlash and wear was the Central of 1935. The patented design held the nut and screw in tight engagement using a flat spring. Slop between the sliding parts was controlled by careful handfitting and use of large hardened bearing surfaces to eliminate wear. You never have to check the vernier to prove the right adjustment was made, just feel the clicks and you know it happened. I have original Central sights from before WW2 which work as well as the day they were made, but other post war Australian designs like Rawson, Lane, Austral and Premier can be as good. The proof of how good the Central design works is that it is still in production today for shooting the much more accurate .308 target rifles.
Parker Hale eventually made changes to control backlash of the sight slides by adding spring loaded balls behind the dovetails of the elevation mechanism. Later, Wilkes designed backlash control on both wind and elevation into his sights. Modern smallbore sights like Anschutz, Walther and Feinwerkbau all have backlash control designs.