Scope mount question

[dmy]

I have an AR with an integral A2 fixed front sight and picatany rail on top of the receiver (ie: not an A2 rear sight/handle). I am eyeing a 1x? scope which will probably have a 20mm - 24mm dia. opening. I am wondering what height mount I need to achieve: (1) absolute co-witness (ie: front post aligned with the cross-hairs); (2) 1/3 co-witness; or (3) no co-witness (post below the sight picture). I want to avoid bringing my upper to the lgs or I might buy online. Thanks for your assistance.

[kcheung2]

It's personal preference, some people like absolute while others prefer bottom 1/3 cowitness. Your 3rd option for a red dot or 1x optic would raise the optic pretty high, it could be tough to get a cheek weld.

Some optics like ACOG or Lucid have the mount built in so you don't have a choice in height.

[dmy]

Thsnks. I was actually looking for the mount height in mm for each type of sight picture.

[kcheung2]

Actually most mfg's don't break it down by mm but by description. You want absolute cowitness then chose "absolute cowitness" in the description.

for example:

https://www.midwayusa.com/product/2344361021?pid=804367

https://www.brownells.com/optics-mounting/electronic-sights/mounting-hardware/trijicon-rmr-low-mount-right-hand-sku100005762-40217-74985.aspx

[DrewTheBrave]

What kcheung2 said. They are usually identified by "absolute" or "lower 1/3" cowitness. It's really all personal preference. I like lower 1/3 when the front sight is fixed, and absolute when it has a flip up front sight.

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[kenl]

OP, co-witnessing works with non-focusing optics (red dots), but not with focused ones, even if they are 1x. For example, I can not co-witness through my 1x Vortex Spitfire prism scope, but can through a Bushnell trs-25.

Variable powered scopes (1-4, 1-6, etc) have the same problem. This post from the optics talk forum http://www.opticstalk.com/prismatic-...opic41432.html , talking about a 1x prism optic, says it better than I can.

"...
Leupold Prismatic is a 1x scope that is robust, compact and works just fine without batteries owing to conventional etched reticle. It largely competes against red dot sights, which are non-focusing sights. Hence, you can co-witness the iron sights through a red dot. The Prismatic is a focusing sight, so though it is 1x, it is not a collimator and you can not co-witness with it. That is the downside. The upside is that you can have a great variety of reticles in it and it works great without a battery.
..."

[ZombieLivesMatter]

Some optics like ACOG or Lucid have the mount built in so you don't have a choice in height.

Acog doesn’t have the mount built-in, it comes with a free mount from the manufacturer but it’s not built-in, it uses simple screws. Many companies like LaRue, ADM, and GG&G make Acog mounts, I’m running the LaRue mount.

[land locked]

OP, What scope are you looking at purchasing? It could help to figure out if the heights actually matter.

Example, I have an A2 fixed front, an ultalight aero cantilever mount and a leupold freedom 1-4x20. I cannot line up the cross hair at any power and at 4 power the fs is a ghost at the bottom.

[dmy]

I just ordered a Truglow Truebright 30 which is 1x6x24. It comes with a mount, but the photos look like my A2 post and maybe part of the front sight base will be in the sight picture. I'll try it and see if part of the front sight obscures the sight picture. Thanks all for your help.

[kcheung2]

At 1x you'll get an approximately absolute cowitness, at 2x the front sight will just be a slight blur on the bottom, and any higher magnification than that you won't even see it.

[SloChicken]

OP, What scope are you looking at purchasing? It could help to figure out if the heights actually matter.

Example, I have an A2 fixed front, an ultalight aero cantilever mount and a leupold freedom 1-4x20. I cannot line up the cross hair at any power and at 4 power the fs is a ghost at the bottom.

[Ki6vsm]

Curious though, how can you use your rear sight at all with the scope on? Seems it would cover the sight.

I was going to say the same thing. Any 1x scope I know of, standard or prism, has a 2-4 inch eye relief. In order to use the scope it has to be mounted far enough back on the receiver that you'd have to fold down a rear BUIS to make room.

[land locked]

I never said I used a rear sight. The scope prevents that.

I was going to say the same thing. Any 1x scope I know of, standard or prism, has a 2-4 inch eye relief. In order to use the scope it has to be mounted far enough back on the receiver that you'd have to fold down a rear BUIS to make room.

[Ki6vsm]

I never said I used a rear sight. The scope prevents that.

I see. Most times, people would want absolute co-witness of, say, a red-dot so that they can co-witness their irons with the RD. Since you don't care about that (don't care about the front sight), you may as well put on anything that centers the scope above the front sight post, which would otherwise be useless to you and in the way. I can't imagine why you'd want absolute or the "no cowitness" you mentioned.

Also, it's not the "opening" that will determine the height above the rail. It's the outside dimensions of the scope, added to the height of mount mount, that is the issue. If this 1x scope is tubular, go by the outside diameter. If it's a prism scope, that dimension can be specific to a particular brand and model depending on how it was designed. So you need to look into that prism scope's specs and find out what the vertical height is.