Rear sight base gap

[JonathanE]

I have a "bargain" ar15 upper I picked up some time ago. There is a gap between the rear sight base and the carrying handle. This gap is about .013" when the sight base is straight, but of course the base tensioning spring & ball take up that slack, so that contact is made on the right edge of the sight, and the left edge of the sight has about .023" gap. This results in a visualy obvious rotation of the rear sight.

I had to adjust the windage 12 clicks to the right to get my groups centered.

Are these two phenomena related? I'm considering cutting a "U" from Al stock to fit the back of the carrying handle and epoxying it in place to take up the slack.
Does this make any sense, or an I barking up the wrong tree?

 

[BBBBill]

No, not related. There is a "standard" modification to eliminate that annoying tendancy, but the effect on sight alignment would be so small as to be insignificant on anything but a pure target rifle. I really don't think you would notice it even then.

 

[JonathanE]

Thanks, Bill,
Two follow-ups:

1. Do you think this rifle's windage issues are something uglier (barrel/ upper alignment?)

2. What's the "standard modification" you mentioned?

-Jonathan

 

[BBBBill]

Could be that, a canted front sight base, or a combination. Or a bent barrel or....

The modification is done to the vertical face of the upper receiver. there are variations on the fix, but here'e one way to do it - http://www272.pair.com/stevewag/arsight/arsight.html.

Here's an article by Glen Zediker that discusses it, too. http://www.zediker.com/articles/sights.pdf Scroll down to the bottom of page two where it says "Problems in the elevation assembly:" and read that section. He discusses a couple different approaches. Keep in mind that he is discussing match prepping the gun vs normal use. Even better, get a copy of his book, "The Competitive AR15". It has pics of the mod and is a great resource to boot, even if you are not interested in match shooting. He has a new book out now called "The Competitive AR15: the ultimate technical guide" that I haven't read yet, so no input on that one yet.
http://www.zediker.com/books/books.html

 

[AR15barrels]

2. What's the "standard modification" you mentioned?

Tig weld a little fitting pad on the lower right front edge of the sight base.
Then file it to fit until the sight sits square (no obvious rotation when viewed from above).

 

[JonathanE]

Thanks AR15barrels. It sounds like this isn't my real problem, anyhow.