So, how much is that click worth?
There are many variations of the rear sight for a Service rifle AR type rifle. Much confusion stems from these variations when it comes to making sight corrections. I have put together some pictures and explanations to help you determine just what the value of a click is worth on your rifle.
First, here comes the math stuff. Without measuring the sight radius of your rifle, we already know some things about the standard sight and what it takes to make ½ minute corrections. If you have a A2 type rifle (20” barrel), the windage is ½ minutes. The thread of the windage screw is 36 threads / inch ( front sight is the same ) and the windage knob clicks 10 times / revolution.
Now the math. When the windage screw is turned 1 revolution, the rear aperture moves 1/36 of an inch or 0.0278”. If the revolution is divided 10 times by the windage clicks, each click is 0.00278”. Let’s call this 0.0028” / ½ minute click. Now we know how much the sight moves up or down, left or right to move the shot ½ minute.
On the standard A2 rear sight, the elevation drum has 25 clicks / revolution of a very coarse thread.
To determine the movement of the elevation adjustments measure the distance from the rear sight aperture that is laying down toward the butt of the rifle and the flat behind the rear sight. Extend a dial caliper, place the end on the receiver and lower the caliper to the sight aperture. Repeat it several times until you get close readings.
This measured 0.356”.
Turn the sight up 1 turn or more (but whole turns only) and measure it again. On the finer adjustment sights, 2 turns works with the 1/2 minute sights and 3 or 4 turns for the ¼ minute sights.
This measured 0.511. So, one turn of the elevation moved the sight 0.511” – 0.356” = 0.155”. Since the wheel has 25 clicks, 1 click is 0.155” / 25 = 0.006” or 1 minute.
Aftermarket sights have several different threads. Some are 13 or 20 threads / inch but how do you know what yours is? Do the same type of measurement. If 2 turns raises the sight 0.100” then it is 20 threads / inch. If it is about 0.150” then it is 13 threads / inch. There may be others but these are the most common ½ minute sights.
The problem comes from the number of clicks / turn. Some manufacturers stayed with the standard 25 click wheel while others varied it. Doesn’t seem like too much of a problem until you apply the difference to the same thread pitch of the rear sight.
On a sight with 20 threads / inch, a wheel with 25 clicks will produce a sight movement of 1”/ 20 /25 = 0.002”. This is 1/3 minute.
On a sight with 13 threads / inch, it becomes 1” / 13 / 25 = 0.003” a true ½ minute click.
But there is another way of getting there. Some wheels have 16 clicks / revolution and 20 tpi. This gives 1” / 20 / 16 = 0.003” also. You can see that some checks are warranted to produce some piece of mind. Verify what you have and write it on some tape and stick it to the butt to remind you.
Back to the windage knob. There are a couple of ways to make a ¼ minute sight, too. The most common is to have 2 detent balls in the knob, one offset ½ of the hole spacing. When you click the sight, one ball rides out of it’s detent and the other drops into it’s hole. Essentially, you divide the revolution 20 times instead of the standard 10. (10 clicks was ½ minute, right?) The other way is to make a new screw with 72 threads / inch and keep the standard 10 click knob.
Bob Jones sells a rear apeture for the A2 that is “1/4 minute like the Marines use.” Only problem is that the thread pitch is 64 tpi on the windage screw. Not 36 or 72 tpi. How can this be 1/4 minute? It can’t!
So, it doesn’t seem like a very big deal but did you ever wonder why it took more clicks for you to do center up than your shooting buddy. Knowing the details of your sights answers many of these questions.