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Looking for a gunsmith for an Ar-15 issue


uber n00b
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I'm looking for a gunsmith who can take a look at my A2 upper. It's shooting low. I think the front sight post is an "F" marked height but not "F" marked.


LEGEN...wait for it... DARY!
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You don't need a gunsmith for that. Any budding armorer or AR enthusiast can walk you through it. YouTube can walk you through how to take out the sight and measure it.

First, I guess we should make sure you know you can screw the front sight up (lowering bullet impact) and down (raising bullet impact).

The title of this one seems promising:
What Mac said.

And if it's shooting low, then the front post needs to be lowered to raise the point of impact.

Screw it down and fire for effect. If at maximum low setting your rifle is shooting too LOW, you can either replace with a shorter post, or very easy to grind it shorter with a dremel (have done this with and SKS and an AK in the past). (corrected the bonehead reversal Mac caught.....)

What range are you zeroing at?

I still zero my AR at 25 yards as we did in the Army. The bullet is still on its upward trajectory, and will come down to where it is dead on at 250 yards, if I recall correctly, making your rifle set for "minute of man" for everything from 25 yards up to 300 yards with no holdovers required.
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LEGEN...wait for it... DARY!
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What Mac said.

And if it's shooting low, then the front post needs to be lowered to raise the point of impact.

Screw it down and fire for effect. If at maximum low setting your rifle is shooting to high, you can either replace with a shorter post......
And you were doing so well.....

You got it right in the first sentence, and then accidentally reversed it in the second.

Shooting too high needs a taller front sight.

The front sight chases the bullet.
LOL, chalk that up to early old-timers disease.

No matter how long I have been shooting, when adjusting iron sights, I always "make my calculations" by holding my hands in front of me, left arm extended with index finger up to simulate a front blade, and right hand back by my eyes, with two fingers up to simulate sight blades, and move my hands up and down to make sure I am correct on what direction moving the sights will have on the barrel moving, for any given sight picture.

I try to do this very subtlely at the range, so that nobody observes me and thinks I'm not playing with a full deck.....



Obsessed Member
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For sight adjustments, just remember the acronym F.O.R.S....Front sight Opposite, Rear site Same. In other words, move the front sight the opposite of the way you want the projectile to go, and move the rear sight the same direction you want to shift the impact.
This holds true for both windage and elevation.
For optical sights, move the reticle the same direction you want to shift the bullet's impact.


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As for how much to adjust an iron sight....
Heres how to calculate how much to move a sight, or to add to it or subtract from its height.

Lets say you were working on an M1 Garand that was shooting 9 inches high at 100 yards, and it has a front sight to rear sight distance of 27.875 inches.

A = The distance from the rear sight to the front sight measured in inches.

B = The distance from the barrel of the rifle to the target in inches. Example: 100 yards is 3600 inches , 200 yards is 7200 inches, 300 yards is 10,800 inches, etc.

C = The CHANGE in height required in the front (or rear) sight to change the elevation, or the windage. The amount (answer) will be in hundredths of an inch in most cases.

D = The height ABOVE or BELOW, or LEFT OR RIGHT of the bulls-eye of the target that the bullet is off by, again stated in inches. And assuming of course that the bull's eye was your point of aim.

So in the example, A would equal 27.875 inches, B would equal 3600 inches, C would be the amount we need to add or subtract from the front sight's height, and D would be the amount in inches we need to get the bullet to move.
Actual Relationship

A is to B, as C is to D. Therefore, the formula is as follows:

---- = ----

Restated: A x D = B x C or C = (A x D) / B

So if we plug in the numbers, we get:

C = (27.875 X 9) / 3600 OR C= 250.875 / 3600, therefore,

C= 0.0696 inches which is the amount that you need to lengthen / raise the front sight by.

You can build it up with Hard Solder, but for a quick test use JB Weld Epoxy with stiff layers of masking tape forming the "Mold" to fill into with the epoxy.
Once it hardens (20 minutes, tops) you can then shoot it, file it down if needed, build it up more, etc. all right at the range if you want to.

Once you are satisfied that the sight is now the right height, you can then replace the front sight blade, or weld or solder on an extension, whatever you want as a permanent fix, although the JB Weld will last quite a long time if you don't knock anything against it and break it off.

If you have to shorten the front sight, then use a fine cut file, take one or two light strokes on the top of the front sight, and measure its length, repeat until you are close to the new overall length you calculated, then shoot if to verify.


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Man that's too damn hard.

I'll just move the sight a bit, and blast off a few rounds..... then move it again.....
Yep. I just got a new red dot for my AR and went to Front Sight. When we were adjusting our sights and I was hitting high right. So I crank down a lot and over a bit. RSO comes over and "Oh you need to do this and that"
Ummm yes I already did.
"How much??? You need to do like 3 times that amount". So I was dumb and did.
Test fire... Off the paper.
Screwed me up so much I never did get it really set in. If I didn't do what he said I would have been pretty close second try.

But if I ever do rifle at FS again I will go in with the sights set for head shots. They got pissy at me for adjusting them early the first day because I was getting nut shots. "We will adjust the sights when we tell you too!!!" Figure it would annoy them if all my first shots are head shots.


Obsessed Member
Is the upper receiver a true fixed carry handle upper, the "F" marked front sight base is for a "Flat top" upper with removable carry handle.
you may have to shorten the front sight pin .040" [IIRC], when you use a non F marked FSB with a removable carry hand and flat top upper you need a taller front sight pin. I ordered a + .060" for a upper with a non F marked upper I built from Windham Weaponry...



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The difference in an "F" FSB and a regular FSB is .040" or 1 mm. You can see the difference and where it is measured in the picture below. If your rifle is shooting low and you have an F front sight base on a rifle I would suggest filing about half that or .020" off the front sight pin and screw the front sight down for the rest. That way you aren't changing the sight a whole lot. For the reverse you can buy a front sight pin .040" taller from Windham Weaponry and probably some others.

My experience is that FSB's vary a lot. Some marked F are not much higher than a standard one and some standard ones are darn near as tall as an F.



uber Member
You can also move the sight all the way to one direction divide it by 2 and then you can find the middle. Also do that to your rear sight as well for elevation. Move it all the way to the left and count the clicks to the right divid by 2 and you find your middle. Also find a control zero so you can adjust from that. Front sight zero line rear sight zero line.

After that you can also zero and 36 yards and confirm at 100 yards.

That how I was taught in the marksmanship course if sights are really jack up.