BH,
Is it the zero stop thing that Cecil puts on that removes the slack? If so, it must be a different concept than that of Nightforces in order to accomplish it, because I don't see how it comes into play until the turret is dialed all the way in to the stop? Backlash is a result of a loose thread fit and unless he replaces them in some way, I don't see how it could be corrected.
When you unscrew either turret, it relies on a spring to push the erector tube tightly against the turret as it's backed away from this inner tube. If these leaf springs are weak, (and most say Leupold uses somewhat lighter springs) and drag even a little and stick, the tube may not be moved the calibrated amount at some point upon turret rotation, then all of a sudden it may free up and move a couple or more clicks at once... possibly after recoil from firing the rifle.
The solution most folks make habbit in order to avoid this problem if it exists or not is to unscrew the turret 1 MOA or so past where you intend to set it, then screw it back in the 1 MOA "toward" the spring to preload it... windage or elevation. Lefty loosy, righty tighty... think of the turret as pushing the bullet the direction you want it to go and you never have to look at a turret to know which way to dial. In a in a 9 O'clock wind, you need to push the bullet back left, so the turret gets screwed in to push it back left. In this case, you've already preloaded the spring so dialing 1 MOA past, then back 1 MOA is not necessary. If wind was at 3 O'clock and you unscrew the turret to let the spring push the bullet back right, the spring is unloading and should then be preloaded again, same as dialing out to a farther range unloads the elevation spring.
The best thing to have is stronger springs and ultra precision fit turret threads, which is why Nightforce is known for its tracking and repeatability.
Lock your rifle down in a rest so it does not move when you make turret adjustments and with a grid target, make a thurough check its tracking and repeatability throughout its full range, both turrets. This is the only way you too can truely calculate what your turrets are specifically calibrated at per click, and if that remains consistant throughout the whole range.
Other things to consider, as you were already preloading the spring when you returned to your 100 yard zero, are things like; was the barrel fouled and settled in when you zeroed it, or did it shift slightly as it fouled in while you fired more further out, was your rifle recoiling consistantly, as in same type rest and grip on rifle etc, had your barrel temp become drastically different as you switched ranges, etc...
Definitely will require some time and careful study to be sure of the cause.
