Iron Sight - offset when zeroed

[Krones]

Hi,

I've recently bought a Daniel Defense DDM4v7 (free-float m-lok handguard). I've put iron sights on it... but when the sights are zeroed, the rear sight is completely offset (all the way to the left). I can only zero the sights with the rear adjusted to the max to one side. What could be causing this and how would I fix it? Rifle is new. Picture of zeroed sight attached.
Any idea what could be causing this?

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[illinoisburt]

Looks like the sight is not manufactured correctly. Its almost a straight line down from the current setting to the mid point of the bottom sight base.

 

[Krones]

It may be because the spring wasn't engaged... new picture attached.
I've tried with MagPul MBUS Pro too and same result - had to move them all the way to the side.

 

[grampajack]

I'm assuming the front sight is mounted on the rail? Or does it have an old fashioned FSB up front?

 

[Krones]

Mounted on the rail as well. Similar setup to the attached (different sights though)

 

[grampajack]

Mounted on the rail as well. Similar setup to the attached (different sights though)
View attachment 768474

I haven't taken any of the new DDs apart yet, so I'm not sure how that rail works. Obviously the rail is not lined up with the flat top. I would just call DD and ask to send the upper back to the factory. If they won't handle it, then level the upper in some kind of fixture using a decent level on the flat top, then check the rail. Be sure and level the flat top on both axes.

 

[Skylerbone]

Had a similar problem once with a cheap red dot sight. Turned out I had managed to tighten it down with the adjustable plate (which was asymmetrical) upside down against the Weaver rail. I noticed when firing the sight seemed slanted because it was. Something to double-check.


A mock-up for visual: the left-side plate that tightens against the rail has a short lip and long lip, both of which will tighten against the base, but only one side allows the pictured ring to set level with the receiver top rail.

 

[Art Eatman]

The top of the rail is not centered on the barrel.

 

[FL-NC]

My guess is that either the rear sight is defective, or the front sight isn't centered on the rail because the rail isn't aligned with the barrel. Hard to picture a defective upper receiver housing.

 

[Hummer70]

I have had that happen to me three times. Take a long close look down your barrel from both ends and I think you will find out the internal bore of the barrel wonders off to the side. Had it happen on a Colt HBAR, Rem 40X and a Rem 7615 rifle. HBAR and 7615 barrels were replaced by factory. The 40 X barrel was replaced by me. I have also seen it in a Savage Mod 24 rifle and a shotgun.

 

[grampajack]

I have had that happen to me three times. Take a long close look down your barrel from both ends and I think you will find out the internal bore of the barrel wonders off to the side. Had it happen on a Colt HBAR, Rem 40X and a Rem 7615 rifle. HBAR and 7615 barrels were replaced by factory. The 40 X barrel was replaced by me. I have also seen it in a Savage Mod 24 rifle and a shotgun.

Man, that's downright disturbing. Very surprising coming from Colt. Hopefully that's a thing of the past now that colt has combined their civilian and contract lines.

 

[Sam1911]

I've seen similar things. Had an FAL that looked beautiful and was all matching parts except for the lower receiver, but the rear sight had to be adjusted almost completely off the gun to zero.

A pal got out a straight-edge and we were able to quickly see that the barrel had been installed into the receiver ever so slightly crooked. While in theory an AR should just plain go together dead-in-line, a tiny deflection in any of the machining of the receiver-to-barrel (or in this case, receiver-to-rail) fit could translate to enough deflection between the front and rear sights to give you the problem you have.

If you think about it, sights are adjustable primarily because we can't machine things so precisely that the bore will be perfectly aligned with a fixed set of sights, right off the factory floor. (Also, to accommodate different loads, adjustments for range and wind, of course.) You've just got enough dimension mistakes stacked up in your rifle that you can see a significant offset when the sights are aligned.