View Full Version : AR scope mounting question

06-19-2007, 12:36 PM
Looking to put a scope on my 16" midlength flat top, I believe I will putting on the 4-16x42 nikon. After doing some measuring, it looks at if the scope will only be a few inches away from the front sight post. Now I hear that on a 20" barrel you won't even notice the front sight, but does anyone have this setup on a 16" barrel? Will I run into any problems?

06-19-2007, 12:56 PM
With a 4X scope the front sight seems to disappear.

On my 375 H&H the front sight post fills the bottom third of the scope on 1X but when I crank the power up to 4X I don't see it any more except as a blurry spot if I'm looking for it.

06-19-2007, 1:15 PM
i dont even really notice my FSB when i dial down my 3-9x to 3x

06-19-2007, 1:17 PM
I had a 4-12x scope on a CMMG M4gery upper the other day. The front sight was right in front of the scope, and I couldn't see anything solid. I'm assuming it cut down a bit of light, but it really wasn't obvious at all.

06-19-2007, 1:31 PM
Thanks! Maxicon, what height rings did you use?

06-19-2007, 1:47 PM
With a 4X scope the front sight seems to disappear.

On my 375 H&H the front sight post fills the bottom third of the scope on 1X but when I crank the power up to 4X I don't see it any more except as a blurry spot if I'm looking for it.

lol, saw you on the hunting section @ Bloodydecks:p

06-19-2007, 3:01 PM
lol, saw you on the hunting section @ Bloodydecks:p

I'm all over the net, especially on the California sites. :chris: I've been on BD for years and was onboard the Q-105 when the original "Darkside" was formed back in 2001 but I'm not a joiner.

06-19-2007, 9:57 PM
Maxicon, what height rings did you use?

They're Warne Maxima mediums - 0.46" high. They're not tall enough to center the optic with the iron sights. On the CMMG upper with handguards that are a bit higher than the rail, the front bell barely touches the handguards. For people who like a low cheek weld they're OK, but for traditional AR style cheekweld, they should be higher.

I mis-spoke about the scope - it's a 3.5-10x Leupold LR/t clone, and I was using it at 10x, where the sight is unnoticeable.

At 3.5x, it's a pretty obvious blur, and at 5x, it's still there some.

Note that this is the worst case - the most FSB possible to see in a scope. A higher scope mount would have less blur, and it would be lower in the field of view.

Here are some pix:

Sighting on a fake woodgrain cabinet at 3.5x - major FSB post blur:

At 5x, it's better, but still a lot of blur:

At 7x, there's still a bit, but it's hard to get in the pics. I can see it as I move my head back and forth, but it's just a shadow:

At 10x, it's pretty much gone:

06-19-2007, 10:28 PM
And here's the CMMG upper with a Leupold LR/T clone in Warne Maxima medium rings. You can see the scope axis is well below the FSB post height:

06-20-2007, 3:47 PM
two words...gas block.;)

06-20-2007, 5:44 PM
Ideally the crosshairs on the scope would be exactly at the top of your front site so that your cheekweld is identical. This way it doesn't matter what you are sighting with, irons or scope. The maximum that the front post should cover in your scope view is just below the scope's crosshairs.

06-20-2007, 7:02 PM
two words...gas block.;)

1 word: "Dremmel"!

Ok.... just cut off the |\ from the front sight....

06-21-2007, 1:05 PM
How would you change the front post out to flat....? Is it easy...?

06-21-2007, 2:46 PM
Here are some pix:

Great post! And I don't say that often. :p

06-21-2007, 10:05 PM
Thanks - appreciate that!

Due to some whining over at arfcom, I took some pics of a more realistic setup with higher rings, using some stuff I just picked up.

Interestingly, as the FSB drops in the FOV, the eyebox position has an even bigger effect.

Same upper, but with a Leupold Mk4 PR 3-9x40 in Mk4 high rings - the scope's optical axis is still lower than the iron sights, but not as much so:

Here's 3x at the rear of the eyebox - pretty distinct shadow, but lots less than with the scope clone mounted low:

and 3x at the front of the eyebox - the shadow's not very strong:

Here's 5x, rear of the eyebox:

06-21-2007, 10:07 PM
And 5x, front of the eyebox:

At 7x, it doesn't make much difference between front and rear (and the eyebox has tightened up a good bit), as the blur is mostly gone, though you can see it if you try:

And at 9x, no shadow to speak of:

06-22-2007, 8:27 AM
I'd sooner spend the $$ and get a low pro gas block, or a single rail gas block. But, if you wanted to do it on the cheap, here's the guide from ARFCOM

Originally Posted By FALARAK:
Make your own cut-down low profile gas block, step by step:

First, I started with a FSB that I had to dremel into to get the pins out. It was otherwise ruined, so I figured, why not cut it down???


Get a dremel and a cut-off tool, and start whacking the top off. I didnt even burn up one disc for the whole project!


First step complete:


Now cut off the sling mount and bayo lug, if desired:


06-22-2007, 8:28 AM
Part II

Originally Posted By FALARAK:
Here is what she looks like at this point. Ugly!


The rest will be done with a sanding drum.... or you can use a griding stone. I find the drums, while they burn up quick, go a little faster:


Most of the rough work is done. At this point, just smooth it up a bit to look better:


If you are going to mount your FSB back on the same barrel it came off of.... then re-use your taper pins. If you are going to mount the FSB on a different barrel, you will need to drill and tap for set screws to hold it to the barrel. I then drilled with a #21 drill, and tapped with a 10-32 tap. I also had to taper-grind my set screws.... so they were more "pointy" on the front. Without doing this, they would bottom out against the barrel with hardly any threads, and would strip out. Dont get them too tight or they will trip. Use red-loctite on them so they wont back off.


06-22-2007, 8:28 AM

Originally Posted By FALARAK:
A quick coat with flat black spray paint:


Finished product:


07-15-2007, 10:33 PM
Thanks Maxicon, thats exactly what I was looking for!:37:

07-15-2007, 11:33 PM
A low profile gas block is like $40. I'd spend the $$.

07-16-2007, 6:54 PM
No I get it, but I want to keep the rifle stock and simple. I looking to set up for occasional scope work.