Does any one know the diameter of the large hole arpeture,the close up site on an A2 so I can drill out the A1 arpeture hole to the same size,TIA 927

Buy an A2 aperture and switch it out. I dont recomend it myself.
GG

I believe the large diameter (0-200M) hole is .200".  3/16" would be a little under, and 7/32" would be a little over that.

When you flip the rear sight blade there is a shift from side due to the threads of the screw it rides on.  The two holes are supposed to be offset by that amount to compensate.  It is unlikely you will drill it accurately enough.  Just buy an A2 sight blade.  While you have the adjustment drum off, toss it in the trash, and replace it with a Rapidex Windage Knob.  Also, replace the front post with an A2 square front post.  This give a much sharper image, especially when the light comes from the side.  You will then have sights that are, for all practical purposes at realistic engagement ranges, as good as the A2 sight system.  (ed to add, the Rapidex knob has 1/2 MOA clicks)

The Rapidex Windage Knob from www.dpmsinc.com is part number UR-39A1, $9.00.  From www.bushmaster.com the same knob is LMT-001K at $9.95.  And both DPMS and Bushmaster have replacement A2 front posts and rear blades.

Its a pre ban bushy 1 of 2 so the upper already has the front A2 post and thanks for the info guys much apreciated.

I changed out the rear aperatures on my A1/C7 uppers to the A2 aperatures, then decided I didn't like them. I then custom drilled the peeps on the A1 aperatures to a size I liked, and reinstalled the A1's.

ed; I also did this aperature mod/change on my A3 carry handles and ARMS #40.

Spend the $30 and buy the Express Sights same plane rear aperture.  It has a small hole and large hole, and both of them are on the same plane so when you flip between them you don't change your point of impact.

Redrilling an A1 sight or using an A2 aperture will cause a shift in the point of impact.

The change in bullet impact on the A2 sight is by design. The problem with the A1 aperature is you have to use Kentucky windage at almost all distances. You shoot a little low at 50 meters and a little high at 300 meters.

The A2 aperature allows you to use the same sight picture at all distances out to 300 meters which works with the 62gr ballistics better.

In the Army, we zero our M16s at 25 meters and the point of impact is supposed to be the same at 250 meters.

As the M16A2 is designed to shoot farther than the A1, when you flip the rear sight after zeroing, the point of aim/impact changes from 25/250 meters to 75/325 meters.

The point of aim/impact changes are drastically less with the A2 aperature allowing you to aim center mass at all distances from 50 meters out to 300 meters.

I modified the A1 rear I have an A2 front post,using the original arperture hole as a guide I used 2 sizes .123 and .187 then the final .200 bit on a drill press,it came out perfect,I shot it today it shoots point of aim out to 100 yds,its an 11.5 with 5.5 supressor so its going to be my short range carbine and I saved a few bucks and have a close range battle site.Thanks to all who help4ed..dave