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Discussion Starter · #1 ·
Hey guys,
So i've been taking a look at my geissele mro mounts that i purchased (both absolute co and lower 1/3) and the height from rail to center of optic seems way lower than what i'm used to. My lower 1/3 rail mount is basically an absolute cowitness and the absolute co is around .2 inches lower than what i've seen from other mounts.

What gives? Is there no industry standard when it comes to absolute cowitness or lower 1/3?
 

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Anti-Federalist
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Hey guys,
So i've been taking a look at my geissele mro mounts that i purchased (both absolute co and lower 1/3) and the height from rail to center of optic seems way lower than what i'm used to. My lower 1/3 rail mount is basically an absolute cowitness and the absolute co is around .2 inches lower than what i've seen from other mounts.

What gives? Is there no industry standard when it comes to absolute cowitness or lower 1/3?

You don't mention what sort of iron sights you are using so hard to say. I would imagine the mounts are designed for F marked FSP and it's corresponding rear sight aperture.
 

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Redacted as OP has posted measurements

This thread highlights the importance of looking at the manufacture’s spec sheet to see if optic heights meet your needs.

Not all “lower 1/3rd” mounts are identical in height. Nor have I ever seen anything published stating they were.
 

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Discussion Starter · #4 ·
I was using LMT fixed irons and magpul buis.....
So i got a response back from geissele....as i suspected, their heights are shorter than other companies:


Good Afternoon,

Thank you for contacting us and for your business. They are 1.54” from the top of the pic to the centerline of the optic When it comes to optic mounts, there is no industry standard for lower 1/3rd height aside from the concept of the irons sitting in the lower portion of the optics viewing window. This may result is a range of variances of what a manufacture considers 1/3 height and comparing two different mounts from two different companies may result in a height differential.

Absolute co-witness even has some variance to it depending on manufacturer but is much more universally in line and typically they are all approx. in the same plane. Absolute CW being in the range of 1.42”-1.48” and this can definitely make comparison of ours seem as if it is absolute CW. There is nothing wrong with either height simply different design parameters.

The picture below illustrates the height difference between our co-whiteness and lower 1/3 (and 1.93”). The laser engraving of the Super Precision logo almost looks squished and the lower 1/3 (in the middle) has plenty of spacing.

On the other hand, my larue mounted T2 is a bit taller.....

buyers beware, every company appears to use their own standards when it comes to lower 1/3
 

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Anti-Federalist
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Problem identified... There is in fact an industry standard. It's referred to as "milspec". It's not always optimum but it will align with "milspec" co-witness. Lower 1/3 has no such designation. "Builder beware".
 
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Discussion Starter · #6 ·
Based on my measurements, although i will have to admit i can't be 100% sure i'm at the dead center of the optic, the rail to center of optic height on the geissele lower 1/3 is closer to 1.45 than 1.55.

My larue lower 1/3, which is officially 1.64 height is at least .2 inches taller.

.2 inches is the general difference between lower 1/3 and absolute cowitness. When i searched online, others had also complained (such as on brownells reviews) that their lower 1/3 is indeed virtually a absolute cowitness.

Lesson learned- always measure and test things and don't take anything for granted.
 

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Sucks for you, but learned something new. Highlights the need to look at manufacturers spec sheet for height dimensions
 

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They can also claim that your optic is off "standard".

1/3 always meant somewhere below absolute to me. That is the advantage of absolute- I can always check if my optic has wandered off zero. Most optic mounts do not post measurement specs, unless you dig specifically (often just say low, med, Hi)-- I hate that. Good luck figuring it out. ---Same problem if you mount a flip mount for magnifier, no standard there.
 

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Problem identified... There is in fact an industry standard. It's referred to as "milspec". It's not always optimum but it will align with "milspec" co-witness. Lower 1/3 has no such designation. "Builder beware".
"MILSPEC" is just a minimum. Or is it lowest bidder? So confusing.
 

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"MILSPEC" is just a minimum. Or is it lowest bidder? So confusing.

It's the minimum tolerance allowable using the requisite materials. It could be the lowest bidder as well.
 

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With government contracts, mil-spec is a minimum standard.
Contract are most often given to the lowest bidder that meets or exceeds said standards.
 

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Interesting. So if the "MILSPEC" is a "minimum standard" and/or "minimum tolerance allowable" and those standards and tolerances are high and closely monitored (E.g. batch testing et al.), doesn't that make the "MILSPEC" better than say a product that is not "MILSPEC"? Doesn't that make the made by the lowest bidder joke moot?
 

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Interesting. So if the "MILSPEC" is a "minimum standard" and/or "minimum tolerance allowable" and those standards and tolerances are high and closely monitored (E.g. batch testing et al.), doesn't that make the "MILSPEC" better than say a product that is not "MILSPEC"? Doesn't that make the made by the lowest bidder joke moot?
It's not about "better". It's about modularity and battlefield interchangability.

When discussing the goverment, Milspec is a requirement that must be met or it's not considered, even it's "better".
 

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It's not about "better". It's about modularity and battlefield interchangability.

When discussing the goverment, Milspec is a requirement that must be met or it's not considered, even it's "better".
So there's more? Like I said, so confusing. Since this is the Black Rifle Forum, I offer this:

Mil-spec is the MINIMUM requirement as set by the United States Military. The Military Specification for the M16/M4/AR15 rifle talks not only about SIZE of parts, but METALS used, as well as metal TREATMENTS and HOW the rifle is ASSEMBLED.

The ‘mil-spec’ is an example of what is called a ‘TDP’ or Technical Data Package. Here’s a formal definition:

Technical Data Package – “A technical description that is adequate to support acquisition of an item, including engineering and production, the description consisting of all applicable technical data such as engineering drawings, associated lists, product and process specifications and standards, performance requirements, quality assurance provisions, and packaging details. Acronym: TDP”

Colt Defense still owns the TDP for the military service rifles. The Colt TDP is heavily controlled and contracted out to FN (and soon Remington as they begin making some military M4’s).

Quite simply a rifle that is built to mil-spec is a better rifle than one that is not. Mil-spec refers to much more than just a blueprint showing part sizes and an assembly diagram. AR-15’s are sophisticated weapons from an engineering standpoint and there IS a difference between the bargain rifle and a higher end gun. If you choose wisely you can pick a rifle in your budget with features you want and the quality you deserve.
 

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So there's more? Like I said, so confusing. Since this is the Black Rifle Forum, I offer this:

Mil-spec is the MINIMUM requirement as set by the United States Military. The Military Specification for the M16/M4/AR15 rifle talks not only about SIZE of parts, but METALS used, as well as metal TREATMENTS and HOW the rifle is ASSEMBLED.

The ‘mil-spec’ is an example of what is called a ‘TDP’ or Technical Data Package. Here’s a formal definition:

Technical Data Package – “A technical description that is adequate to support acquisition of an item, including engineering and production, the description consisting of all applicable technical data such as engineering drawings, associated lists, product and process specifications and standards, performance requirements, quality assurance provisions, and packaging details. Acronym: TDP”

Colt Defense still owns the TDP for the military service rifles. The Colt TDP is heavily controlled and contracted out to FN (and soon Remington as they begin making some military M4’s).

Quite simply a rifle that is built to mil-spec is a better rifle than one that is not. Mil-spec refers to much more than just a blueprint showing part sizes and an assembly diagram. AR-15’s are sophisticated weapons from an engineering standpoint and there IS a difference between the bargain rifle and a higher end gun. If you choose wisely you can pick a rifle in your budget with features you want and the quality you deserve.

A bargain rifle can be milspec as well. I mentioned them being built out of requisite materials already.

Milspec is milspec. It either is or isn't. "Better" is subjective and doesn't enter the debate.
 

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A bargain rifle can be milspec as well. I mentioned them being built out of requisite materials already.

Milspec is milspec. It either is or isn't. "Better" is subjective and doesn't enter the debate.
Your first and second paragraphs contradict. Now we have to define what a "bargain rifle" is. After all, "MILSPEC" is "MILSPEC". It is or isn't. Price doesn't enter the debate. In a governmental contract, isn't the lowest bidder inherently making the bargain rifle?

I'm not saying one built to "MILSPEC" is better than one that is not. I think we both know that's not true. Just banter on what and how "MILSPEC" is defined. That's a reason I put it in quotes, because I don't think anyone can agree on a definition.
 

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Your first and second paragraphs contradict. Now we have to define what a "bargain rifle" is. After all, "MILSPEC" is "MILSPEC". It is or isn't. Price doesn't enter the debate. In a governmental contract, isn't the lowest bidder inherently making the bargain rifle?

I'm not saying one built to "MILSPEC" is better than one that is not. I think we both know that's not true. Just banter on what and how "MILSPEC" is defined. That's a reason I put it in quotes, because I don't think anyone can agree on a definition.

I don't have any trouble defining milspec. I thought you did a pretty good job defining above.. :dunno:
 

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Interesting. So if the "MILSPEC" is a "minimum standard" and/or "minimum tolerance allowable" and those standards and tolerances are high and closely monitored (E.g. batch testing et al.), doesn't that make the "MILSPEC" better than say a product that is not "MILSPEC"? Doesn't that make the made by the lowest bidder joke moot?
Mil-spec is a level on a scale
You can have a model above or below this level on said scale. Many have been buying budget priced guns. These are not milspec, but many don’t seem to mind.

Personally, I wouldn’t put anyone down for buying a budget priced model. But, I will admit, I am not a fan of them.

And the “lowest bidder” comment isn't a joke, it’s Government contract Fact.
 
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