All sight settings, whether based on distance to target, ammo, light or wind conditions are based on a repeatable starting place. As sight settings (zeros) change over time, whether due to that mentioned above – or we gain more experience… It is best to index this starting place at what is known as “mechanical zero”.
Mechanical Zero (MZ) is defined as the rear sight bottomed out and the windage knob set directly in the middle, allowing for an equal and maximum amount of travel in either direction – left or right.
This mark is the starting place for elevation sight settings. It become extremely useful when dialing up for different yard lines as it identifies when the wheel has made a complete revolution (typically 24 or 25-clicks – depending on manufacturer).
White Oak Armament makes an elevation wheel that does not require this type of marking. It comes as two pieces, where the top half indexes with machined-in numbers that identify every 5-clicks (0-5-10-15-20). An upper purchased from WOA comes with this wheel already indexed to MZ.
The supplied hashmarks on an AR15 are typically too fine (close together) to really tell small adjustments like 1/4 or 1/2-minute clicks. While still a good indicator to identify if the sight is spun a revolution one direction or the other, it is best to stripe the windage knob itself to ensure total repeatability.
As you can see in the example below, one click right or left of MZ can be too difficult to discern only looking at the hashmarks. While an off adjustment like this might not print too obviously on reduced targets (100-yards), the further out the target – the more each click affects group location.
Typically, National Match Service Rifles shoot at 100, 200, 300 & 600-yards. All but the 600-yard setting are well within (or just past) the first revolution of the elevation wheel. The 600-yard elevation sight settings are commonly somewhere within the third revolution. With the rear sight spun up three full revolutions, this stripe serves as a good point of reference that the sight is at either it’s 48th or 50th click from bottom – depending on manufacturer.
• The photo in the middle is the rear sight spun up one revolution.
(It shows a gap of light, but does not match up the hash marks.)
• The photo on the right shows the rear sight bottomed out (or close), set at MZ.
OTHER MARKING AREAS:
Leaf & Screw-In Hooded Aperture:
No matter how well machined these hooded sight inserts are, even from the best manufacturers – their apertures are never perfectly on center. Should they come loose, groups on target will definitely open up and go off-call. A simple stripe across the hood when tightened snug onto the leaf will suffice as an indicator. Alternative hooded apertures can also be striped for quick changeout as range conditions require.
After assuring your windage adjustable front sight base prints as close to MZ as possible and has been locked down accordingly, place a stripe here – fore and aft across the sight base. Should the rifle ever take a spill to the shop floor or be jarred in transport, this line will reveal if the sights have moved. Nail polish is good for this as it will create a seal that will easily be noticeable should it be broken.