Rifle Magnified Optic Mechanical Zero and Tracking

wormydog1724

wormydog1724

Member
Stay with me here, this is going to take some explaining.

So I recently stumbled across a post on a far away forum about finding an optic's mechanical zero, where the crosshairs are in the middle of the optic, using a Mirror. Why would I want to find the mechanical zero before mounting and zeroing the rifle? I don't know, just thought it was cool.

Basically it goes like this:

Set mirror on flat surface
Stand optic on objective and look down into the optic
You should see two crosshairs, one is the true crosshair and one is the mechanical zero of the optic
Make elevation and windage adjustments until both crosshairs are identical
Scope is now at mechanical zero

I hope that makes sense, it did for me but I really understood it when I tried it.
The other method is to bottom out the adjustment, count the clicks to the other bottomed out position, then divide by 2. That works and has been used forever, takes awhile and requires me to able to count, which I have trouble with anything over 10. So the mirror method was quicker and easier for me.

I just got a Trijicon 4-16x50 Accupower used and I tried this method to set it back to zero. It was easy and quick.

Then it got me to wondering about using the mirror to perform a tracking/box test. This reticle is a MilSquare reticle.
So I set up the optic on the mirror as before, at it's mechanical zero
Then I did one full turn on each adjustment knob (50 clicks, .1 mil per click) and looked at the crosshairs, the two crosshairs were right on the 5 mils hash. PERFECT
Then I did two full turns back the other way, again they were right on the 5 mil hash marks on the opposite direction.
Then I did a full turn back to zero and sure enough, the crosshairs were back inline and thus back at zero.

I did this several times, all directions. I did notice that if I did two full turns, 100 clicks, 10 mils, that it was just slightly off. My long range shooter friend said even the best scopes can be as much as 3% off which isn't noticeable until large adjustments are made. I have no idea if mine was 3% or more or less, I could just tell it wasn't exactly on the 10 mil square.

Does all of that make sense?

Basically use a mirror to find mechanical zero, do a box test.

Obviously the box test would only work with a mildot or similar type reticle. A plain duplex reticle wouldn't work because there's no reference points on the reticle.

I'm not sure how useful or practical this is because it's just referencing off the optic itself, but I will say I have a disgustingly cheap airsoft type scope that is mildot and it didn't track for crap. It's just on my .22LR upper.

Thoughts? Opinions? Waste of time?
 
I suppose looking in the mirror will center the crosshairs in your objective lens. The mirror reflects back precisely inversely.

This would not necessarily mean you are mechanically zeroed, as the housing could be off center or whatever.

So counting clicks would be best method for getting a truer mechanical zero, as well as knowing how many adjustment clicks your optic has.
 
I've been using that method for a very long time. It is a very quick way to get a scope centered when you are mounting scopes into windage adjustable mounts, making custom rings or D&Ting a rifle for scope mounting. That way you can have everything straight to start with and just fine tune with the scope adjustments.
 
Originally Posted By: AWSI've been using that method for a very long time. It is a very quick way to get a scope centered when you are mounting scopes into windage adjustable mounts, making custom rings or D&Ting a rifle for scope mounting. That way you can have everything straight to start with and just fine tune with the scope adjustments.

I know the mirror method isn't new but have you ever heard of or tried doing a box test with it and then repeating the box test with live fire?

I was surprised and a little unsurprised that it worked. I mean Trijicon is quality so it should perform like it did. I just wonder how much it's worth and if it correlates with a live fire box test. Like I said though, I think the box test mirror method is only verifiable with a mil-dot reticle.
 
Maybe the mirror zeroing should be an optical zero and the turret counting method is a mechanical zero. If full adjustment is 100 clicks (50 up and down from zero) than you should be less than 2 clicks from zero to stay less than 3% tracking error.
 
WD., I use a magnetic Bushnell Grid Boresighter to check tracking of my optics. This system allows me to get approximate scope zero, calculate true turret click value, reticle subtension, reticle movement with magnification and parallax change, consistency of clicks and return to zero. One of the handiest gizmos I have, IMO. I have one older high-end optic that's off on turret click value by ~2X advertised. Really no problem as it's just factored in (for hunting, not comps.). Sounds like your system oughtta' work OK for return to zero and turret click calculations. Plex reticle calcs should work also, once the subtension is known.
 
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