AR15 Scope Mount Height with Fixed Front Sight

[azrocks]

I've never used an AR with both a magnified optic and a FSB.
I'm currently looking at a 1-6 LPVO packaged with a cantilever mount.
The rifle would be a BCM flat-top (A4?) with an 'F' fixed front sight.
The docs for the mount says it places the center of the optic 1.4" above the receiver rail.

My questions:
Would this combination work?
Will it clear the front sight?
What will the co-witness be?
How will the front sight impact the sight picture?

Thanks in advance!

 

[Hookeye]

Its worthless

View attachment 929395

 

[badkarmamib]

I am running a Vortex 1-8 with a FSB. At 1X and 2X, you can see the front sight in the bottom 1/2 of the image. Anything higher, and it disappears, unless you TRY to see it. Also, you CANNOT co-witness with a magnified optic.

 

[azrocks]

I appreciate the reply, Hookeye. Thank you.
My question, however, wasn't whether or not a scoped AR was useful. Obviously it is from your pic
What I'd like to know is if this combination of parts allows the scope to adequately clear the front sight post, where the reticle center will co-witness with the iron sight post, and if the presence (if present) of the front sight in the scope's field of view will impact the sight picture.

 

[Hookeye]

Above 3x the FSB fades out.
2-7X scope, run from 4-7X

Under 3X you see the FSB and it's annoying.
Enough that we never messed with worrying about co-witness or not.

We....my bud and I (that's him with one of my old rifles).

Under 3X, I'd go with a fold down or removeable front sight.

My FSB rifle wears a reddot that does cowitness. Lifting head slightly lifts the dot off the front sight.

A scope will probably not allow that.

Scope and irons in same line, I think clocking the irons a better idea.

 

[azrocks]

So are y'all saying that no matter what the height of the mount is it won't pose a problem? Doesn't the mount have to be a certain height in order to clear the front sight? For all I know your mounts are significantly higher than the one I'm looking at.

 

[azrocks]

Above 3x the FSB fades out.
2-7X scope, run from 4-7X

Under 3X you see the FSB and it's annoying.

The scope I'm considering is a true 1x scope. Being able to run it at 1x is important. This sounds potentially problematic.

 

[Hookeye]

If you want the scope to not see the FSB, you'll be way high and need some cheek riser.
I had an XM15 Bushmaster w carry handle way back and slapped a 4X on the carry handle.
Got the tele cheek piece too. It worked.
Shot a few chucks with it.
And took all that crap off.

A flattop is THE way to go.

 

[Hookeye]

I bet most running a 1X do not run fixed front sight.
My RDS cowotnesses with my front sight.
Doesn't bother me.
But...........like posted before, a slight head lift and the dot is off the front sight. Kinda like to run it that way.
Also, since it is a reddot, the illumination draws my eye to the dot........helps ignore the front sight.

 

[Hookeye]

Sorry for not answering your question directly, I just work around the FSB with an optic.
Higher mag scope, or a RDS...........don't have anything set up for in between.
Both have worked for me.

I suspect your chosen scope type will as well.
Just depends on what you like and or how stubborn you are with preferences.

If you are new to the platform and or optics you probably can adapt easily.

 

[AlexanderA]

Usually the front sight tower is not a problem with an optic, because it's out of focus.

If you're really worried about this, there's a solution: https://www.brownells.com/rifle-par...flip-up-silhouette-front-sight-prod26728.aspx

This is a pricey, and probably unnecessary, solution.

 

[azrocks]

Thanks guys. @Hookeye, no apologies necessary for not answering my question directly! You've provided a ton of input, which I greatly appreciated!

@AlexanderA, thank you as well. Its not that I'm worried about it necessarily, it's that I have absolutely no idea what I can expect to see when looking down the scope or how it will (or won't) impact functionality, particularly on 1x. I don't care if I see the front sight base, so long as seeing it doesn't impact my ability to quickly put shots on target. That's what I don't know. Specifically, the scope is a Vortex Strike Eagle 1-6, with an illuminated and etched reticle.

 

[sarduy]

I've never used an AR with both a magnified optic and a FSB.
I'm currently looking at a 1-6 LPVO packaged with a cantilever mount.
The rifle would be a BCM flat-top (A4?) with an 'F' fixed front sight.
The docs for the mount says it places the center of the optic 1.4" above the receiver rail.

My questions:
Would this combination work?
Will it clear the front sight?
What will the co-witness be?
How will the front sight impact the sight picture?

Thanks in advance!

You are going to see something like that.

this is an Acog 4x fixed magnification. The more zoom the less you’ll see it.

 

[MistWolf]

Doesn't the mount have to be a certain height in order to clear the front sight?

Yes. Yes, it does. If you insist on using an FSB with a scope (not a good choice) you'll need to use a scope mount that's what they call "1/3 lower co-witness" (or higher) to get the center of the reticle above the FSB. (I don't know what the actual measurement is.) At higher magnification, you'll be able to see around the FSB (how that works, I don't know), but not see through it. Scopes don't let you violate the laws of physics.

You could get away with using a mount that's "absolute co-witness" which will place the center of the reticle at the top of the FSB sight post, but will limit your usable elevation adjustments and holdovers because, as I stated above, you cannot see through the FSB.

 

[chicharrones]

Here's a few pics. Leupold 1-4x mounted on an Aero Extended scope mount with 1.5" centerline height and F-marked front sight.

1x on the left (actual 1.4) and 4x on the right. The camera made the front sight look clearer than it does to the eye. Co-witnessing doesn't matter as the optic makes the front sight virtually unusable. Just align the reticle as normal and shoot the gun. Looks weird, works fine.

View attachment 929598

View attachment 929599
That gun is a 16" barreled mid-length gas carbine with an A1 stock. The front sight is further away from the shooter's eye than it would be for a typical carbine, but closer than it would be for a typical rifle.

And yes, I tried the set up with the scope moved forward so the rear sight could flip up behind the scope. Still couldn't use the iron sights even with the scope at the lowest power. Too blurry.

View attachment 929600

 

[AlexanderA]

The issue is the focus, not the magnification power of the optic. What the optic does is focus on the target (i.e. "infinity"), with the reticle superimposed on that plane. The front sight is (or should be) completely out of focus, and therefore invisible.

With a camera, you can increase the depth of field by stopping down the diaphragm. This could theoretically make both (relatively) near and far objects visible. But gun optics don't have a diaphragm, and run fully open. This is why you have minimal depth of field (which is actually desirable in this application).

The camera made the front sight look clearer than it does to the eye.

Exactly. Looking through the camera, the depth of field is increased because of the camera diaphragm. The naked eye doesn't work in the same way.

 

[azrocks]

Thanks all. I sincerely appreciate the input. It cleared everything up for me. I also contacted a couple manufacturers that confirmed the 1.4" height would get the scope centerline above the top of the front post. Realize it's not the most optimal sight picture, but it doesn't appear to be much different at 1x than using the irons alone would be in terms of fov restriction, and it obviously goes away at higher mag. Pretty sure this will fit my needs. Got new toys coming so will find out soon. Really appreciate the pics too. That really helped.