Keystone Accuracy

Rear Sight Pin Modification

Many new owners of the AR-15 A2 service rifle notice that the rear sight rotates clockwise with a spring loaded side that returns the sight to its original position.  Those same owners often wonder if this will effect repeatable accuracy and ask how the rear sight can be modified so that there is no movement at all, ensuring that the sight will always return to "zero".  There certainly is a way that was developed by the Army Marksmanship Unit but later perfected by John Holliger of White Oak Precision in the late 90's.  The task is very simple if you have the right tools, but a bit time consuming in order to achieve a close tolerance "hand fit" result.

After the modification is performed, there is absolutely no radial movement in the rear sight ensuring no "zero" shifts from left to right which can be quite frustrating.  It is essentially set up to perform precisely like a precision match rifle sight costing hundreds of dollars while providing extreme durability under the harshest conditions over the life of the rifle.

Below are a set of pictures that allow a visible understanding of the installation and mechanical application of the modification.

Please note that I would prefer to perform this modification on a good quality hardened sight body only.  Soft metal just doesn't make for a good long term fix.

The cost for this modification is: $85.00
Pin & install new CLE NM 1/4 moa sight: $185.00
~ Rock River sights available on request ~
plus return shipping (insured) & the cost of any parts if needed.

~ All listed prices reflect a 3% cash discount ~

(CLE makes the very best NM rear sight in the industry, but if you prefer another brand, with the exception of Northern Comp, please specify at the time of order.  Prices may vary among the different brands)

Once the rear sight is disassembled, the sight is removed and the receiver is put in a precision mill vise and indicated so that all of the center lines of the the sight and the mill spindle are lined up perfectly.  This is referred to as indicated to "zero".
After the receiver is properly indicated, the rear sight is placed back in the receiver and squared with a feeler gauge.  It is then drilled in place from a known distance from the center line of the sight bore with a carbide drill due to the hardness of the rear sight body.
The sight pin hole has been drilled undersized and now the hole must be reamed with a carbide reamer to size in order to get a precise hole diameter in order to accommodate a precisely ground pin.
We now have a properly indicated hole in order to install the first machined pin that fits the hole with a close tolerance sliding fit.  The pin is pressed into the aluminum receiver with an interference fit of about .002-.003".  This pin is now complete and we move on to the rear sight pin.  This is were things get slightly more complicated.
With the sight installed back in the receiver and the front pin engaged in it's freshly machined home, we then drill the rear pin hole through the sight body and receiver, again using a carbide drill.  The rear hole is drilled at a calculated offset due to the odd number (25) of elevation detents in order to install an additional elevation ball and spring in order to keep the equal spring pressure on both sides of the sight body.  This offset will be more apparent in a later frame.
Like the front pin, we must now ream the rear pin to exact size with a carbide reamer.  The reamer is only run into the sight body but not into the receiver in order to achieve the same pressed interference fit that the front pin has.  You may notice that the sight body has been slightly elevated for this purpose.
Once the hole is sized, the sight body must be removed again in order to drill a new spring pocket for the rear sight ball and spring.  This hole is undersized from the pin hole, but slightly over sized in relation to the spring and ball. 
Note:  The additional ball and spring is recycled from the front of the sight body when disassembled, since this ball and spring have no other purpose when removed.
Now we place the rear sight body back in the receiver and press the rear pin into the receiver taking careful note that the pin must move freely in the sight body itself.  If the pin is driven improperly or the pin does not have clearance in the sight body, get ready to hear some very colorful language.  This is a very important step in the process!!
Here is the rear pin installed in the sight body and receiver.  This picture clearly shows the calculated offset of the rear pin in order to install the additional sight ball and spring.  It must sit flush or just a few thousands of an inch below the sight body so that it will never interfere with the rear sight leaf in it's downward position.
All done!  you cannot see it in this picture, but there is a very thin gap between the sight body and the receiver carry handle that is the same on both sides.  This gap will never change and is evidence of a job well done.
With the sight in its most downward position, the last step is to re-zero the elevation wheel so that it stops on the last positive detent and install the spring and pin.  It is very important to note that the sight should ride up and down the newly installed pins with no resistance in order to let the elevation spring do its job.  Some lacquer thinner will make quick work of removing the blue layout die.
I hope that this has provided some insight on how improved mechanics can certainly help the accuracy of your rifle.

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