AR-15 shooting way low with irons

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AR-15 shooting way low with irons

Post by FF1009 » Sat Dec 27, 2014 3:05 pm

Hey guys.
My wife just finished her first AR and we took it out to sight in the irons a few days ago. Right off the bat it was shooting way low (2' at 25 yards). I cranked the front sight all the way down and it is still shooting about 20" low.
It is a 16" carbine with an "F" marked front sight, and a Ruger rapid deploy rear sight.
Any ideas on how to fix this?

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Re: AR-15 shooting way low with irons

Post by Kommander » Sat Dec 27, 2014 3:51 pm

The first thing I would do would be to try to isolate the case by trying the upper on another lower (unlikely that this will solve anything but it won't hurt to check) and trying a different rear sight. Is the Ruger rapid deploy rear sight designed to work with F marked front sights? Also you mentioned that she built it. Did you assemble your own upper or buy a new pre assembled one and if so from who?
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Re: AR-15 shooting way low with irons

Post by jor-el » Sat Dec 27, 2014 5:02 pm

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Imagine a straight line going along the sight axis. Now imagine another line going along the bore axis. That barrel about 2 inches below the sights? In our CQB training using the AR/M16M4 we were taught inside 25 yards you aim at the top of the head for head shots due to the difference between sight line and bore axis.

When you do another test fire, take the upper alone and bore sight the barrel to a target(place the upper in a stable bedding and look straight down the bore). Then look down the sights to see if the sight line agrees with the bore line. Doesn't have to be perfect, just has to be close.

2 feet to 20 inches low at 25 yards. I'm leaning towards a bore obstruction at the muzzle to deflect bullets that far off at short range. What and how large are the groups at that range?
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Re: AR-15 shooting way low with irons

Post by FF1009 » Sat Dec 27, 2014 5:38 pm

Kommander wrote:The first thing I would do would be to try to isolate the case by trying the upper on another lower (unlikely that this will solve anything but it won't hurt to check) and trying a different rear sight. Is the Ruger rapid deploy rear sight designed to work with F marked front sights? Also you mentioned that she built it. Did you assemble your own upper or buy a new pre assembled one and if so from who?

Supposedly the Ruger sight is supposed to work with F marked sights. She assembled it all, nothing seems out of line on the lower. I will put my carry handle sight on and see if the problem persists.

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Re: AR-15 shooting way low with irons

Post by FF1009 » Sat Dec 27, 2014 5:46 pm

jor-el wrote:Image

When you do another test fire, take the upper alone and bore sight the barrel to a target(place the upper in a stable bedding and look straight down the bore). Then look down the sights to see if the sight line agrees with the bore line. Doesn't have to be perfect, just has to be close.

2 feet to 20 inches low at 25 yards. I'm leaning towards a bore obstruction at the muzzle to deflect bullets that far off at short range. What and how large are the groups at that range?
I have inspected the bore and the muzzle device, I don't think anything is causing a deflection. It shoots consistent groups, just really frigging low. I haven't shot groups for accuracy yet, I kinda want to resolve this issue before burning any more ammo.

I have it bore sighted and there is a noticeable difference between the boreline and the point of aim (bore is lower than point of aim) One thing I noticed is a slight leftward cant on the front sight base. I compensated with the rear windage, and there is no gas port alignment issues so I thought it was no big deal.

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Re: AR-15 shooting way low with irons

Post by Doctorr Fabulous » Sat Dec 27, 2014 7:31 pm

Crooked gas block on an AR? Oh boy.

So, when you LBS it, what distance are you LBSing at?

Who built the upper?
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Re: AR-15 shooting way low with irons

Post by JeeperCreeper » Sat Dec 27, 2014 8:07 pm

Have you tried turning it off and turning it on again?


Lay it on a piece of cardboard, get a yard stick, and trace the line from the bore and trace a line from the sights. it will be easy to see if the sights are off by comparing the angles from the two lines as opposed to tearing everything apart. It's not 100% scientific, but it will be a start. Just my 0.02
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Re: AR-15 shooting way low with irons

Post by FF1009 » Sat Dec 27, 2014 8:23 pm

Doctorr Fabulous wrote:Crooked gas block on an AR? Oh boy.

So, when you LBS it, what distance are you LBSing at?

Who built the upper?
I bore sight at 25 yards, but I don't use a laser. The upper receiver is an Anderson mfg and the barrel is a Stag M4 profile.

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Re: AR-15 shooting way low with irons

Post by manowar1313 » Sat Dec 27, 2014 9:26 pm

Have you tried teaching your wife to aim higher?
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Re: AR-15 shooting way low with irons

Post by WutsFrequencyKeneth » Sat Dec 27, 2014 10:34 pm

Don't waste your time or ammo on another lower. There is literally nothing that could change. Everything involved in this scenario is contained within the upper. Barrel, sights, mounts. How about a pic of the upper? That sometimes solves things real fast.

My first thought was to ask if you had a front sight on a railed gas block, but you said you have an F-height sight gas block.

Ruger even seems to use it on their rifles which have an F-height milled gas block: http://ruger.com/products/ar556/index.html

Pics required, I'd say. Also - verify that you are absolutely sure it's an F-height sight. You don't have some sort of upper receiver with the higher-height rail, do you? Haven't replaced the front sight post?

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Re: AR-15 shooting way low with irons

Post by FF1009 » Sun Dec 28, 2014 10:52 am

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Re: AR-15 shooting way low with irons

Post by FF1009 » Sun Dec 28, 2014 10:59 am

Sight base is F marked for sure, the upper is an "A3" and was not advertised as having a variance in the rail height. We did have to order a sight post for the barrel, but it is a standard height.

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Re: AR-15 shooting way low with irons

Post by mystic_1 » Sun Dec 28, 2014 12:05 pm

FF1009 wrote: One thing I noticed is a slight leftward cant on the front sight base. I compensated with the rear windage, and there is no gas port alignment issues so I thought it was no big deal.

To my mind, ANY misalignment of the front sight is a sign of a problem and I'd be looking very closely for the cause of this.

The amount to which you've had to offset the rear sight to "compensate" is alarming to my admittedly amateur eye.

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AR-15 shooting way low with irons

Post by FF1009 » Sun Dec 28, 2014 3:59 pm

mystic_1 wrote:

To my mind, ANY misalignment of the front sight is a sign of a problem and I'd be looking very closely for the cause of this.

The amount to which you've had to offset the rear sight to "compensate" is alarming to my admittedly amateur eye.

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The cause looks like it was not pinned correctly. I got the barrel on a trade, I have kinda deduced that there was some undisclosed history to the barrel. The pins look larger than the taper pins that I am used to and one has a slot in it like it could accept a flathead driver. All things considered though I only had $25 tied up in my trade item, so I get what I pay for. I figured since the gas system was working good and I still could find windage it was alright. Starting to think there is something going on with the barrel itself to cause this whole problem.

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Re: AR-15 shooting way low with irons

Post by jor-el » Sun Dec 28, 2014 6:03 pm

Anyone near you that could check the barrel for straightness issues?
If its warped but otherwise accurate, that barrel should be relegated to an optics only setup.
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Re: AR-15 shooting way low with irons

Post by WutsFrequencyKeneth » Sun Dec 28, 2014 6:26 pm

If it's not shooting terribly far left/right, it isn't going to be an issue with canting of the block. Unless I missed a post, the OP mentioned it shooting low. This makes it sound like the front post is too high, given how high it's shooting. Whether it's the block in relation to the rear sight or if the post is some oddball tall post, I can't tell. A bent barrel would be strange... putting a straight-edge alone the outside should be good enough to see anything obvious. You'd also be able to tell quite quickly by looking down the bore. Really strange that it'd be that far off and it be a bent barrel. Wouldn't think it'd be accurate at that point, and that it'd start effecting the projectiles. Just seems like it must be some mismatch in the parts.

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Re: AR-15 shooting way low with irons

Post by LJ126 » Sun Dec 28, 2014 8:09 pm

Could be a handful of different things. If it were me, I'd try to investigate the easy stuff first. Do you have another rear sight unit that you could swap/borrow to eliminate that variable?
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Re: AR-15 shooting way low with irons

Post by eeb » Sun Dec 28, 2014 8:26 pm

You can cause a pretty stout cant in the front sight when torquing down the barrel nut. I've done that on at least one of mine (the one with the conventional front sight/gas block, probably the others too, you just can't tell because they use lo-pro gas blocks and scopes).

Does the existing rear sight feature height adjustment as well? An A2 style sight may help your situation.
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Re: AR-15 shooting way low with irons

Post by praharin » Mon Dec 29, 2014 11:43 am

If the sight it canted left or right it's also moved lower, which would make you shoot high, not low after compensating got the cant in windage. If bottoming out the front post only brought you back up 4" and the shooter is not the issue I'd start to suspect the muzzle. Poor chrome lining or a bad cut/crown maybe?
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Re: AR-15 shooting way low with irons

Post by JeeperCreeper » Mon Dec 29, 2014 11:54 am

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Re: AR-15 shooting way low with irons

Post by mystic_1 » Mon Dec 29, 2014 1:37 pm

I don't think the cant in the front post is so much causing the vertical deflection in POA vs POI, but it's a sign to me that something somewhere is hinkey. I wouldn't write it off as irrelevant so quickly. Having said that, consider the following:


Assuming a 14.5 carbine sight radius, and given 900 inches in 25 yards, it takes a (14.5/900) = 0.0161" movement of the front site post to result in a 1 inch change in POI. Multiply that by your initial 24" offset and that means the front site was too high by 0.386 inches (or the rear sight was too low by 0.386 inches).

0.386" is about just over 3/8 of an inch. In terms of sight alignment, that's a LOT.

With four click per revolution on an A2 front sight post, and a 36TPI thread, you end up with ((1/36)/4)= 0.00694" movement per click. So you'd need (0.386/0.00694) = about 55 clicks to correct this. No wonder you couldn't adjust it out! :D

Now, if your barrel were bent by 3/8 inch over a 16" length, you'd be able to find that with a straight edge pretty easily. Also, the pinning problems on your FSB surely can't be introducing a 3/8 inch error in the height.

Assuming nothing else in the rifle is off by nearly half an inch, we're back to the question of sight compatibility. Have you tried your carry handle sight yet and if so what were the results? Have you measured the height of the carry handle sight and compared it to the Ruger sight?

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Re: AR-15 shooting way low with irons

Post by MacAttack » Tue Dec 30, 2014 2:10 am

Disassemble the upper and then put it all back together.

Use this time to check the barrel to see if its straight.

Maybe something went hinky during the first assembly. Who knows but anything is possible.

If a cleaning rod close to the size of the bore can go down the barrel pretty easy then it can't be bent to much, at least not enough to notice just looking at the barrel. And definitely not enough to notice visually at the front sight.

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Re: AR-15 shooting way low with irons

Post by emclean » Tue Dec 30, 2014 9:59 am

if it is cycling fine, you could always file down the front sight to match point of impact.

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Re: AR-15 shooting way low with irons

Post by Mikeyboy » Tue Dec 30, 2014 10:38 am

Laser bore sight it. If your point of impact matches where the laser dot is at 25 yards, something is wrong with the sights. If you look down the sights at 25 yards and the laser dot is close to where you are aiming (beside being about 2" low like jor-el mention earlier) then something is wrong with the rifle, barrel, or ammo.

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