A discussion on scope tracking from the guys at Applied Ballistics
Why a Tall Target Test is essential:
Last time I posted about the differences between a MOA and Mil, and how they are Angular Measurements. This week I am going to use that information to explain to you why you must always do a comprehensive tracking test on every scope.
Let’s start by going into how most scopes work. In simple they are a configuration that can be imagined as a bolt and a nut. As you turn the turret the bolt rotates up and down this nut. However, when we talk about a scope turret we are taking about asking for absolute precision. With each click of the turret the user is asking for an adjustment of: 0.00415 degrees (1/4 MOA clicks), 0.002075 degrees (1/8th MOA clicks), 0.00573 degrees (1/10th Mil), and 0.002865 degrees (1/20th Mil). This requires absolute precision. Now consider the calipers most of us have as reloaders. If you have spent a couple hundred dollars on a high-end set like Mitutoyo or Starrett they often come with a card that tells you their “calibration” or how far off, you can expect them to be. These numbers are often “repeatable” but not “perfect”. Your scope is the same way. It is often repeatable but not always absolute precision. So, to counter this, you will need to perform a Tall Target Test. The results of the test will give you a scope calibration number. You can multiply this number times you ballistic firing solution and get the corrected firing solution. Or you can input this as a SSF (Sight Scale Factor) into much of our software and it will do it for you.
To put what is going on into perspective lets do a visual exercise. Remember those protractors you had in high school. Generally they were 180 degrees. 1 Degree is 60 MOA. So break a single degree off that protractor into 240 parts (Majority of scopes are ¼ MOA clicks and 60 * 4 = 240). That is what you are asking the scope to do. Or you can try to image what the 0.00415 sliver of that degree looks like off the protractor.
Tall Target Testing is done by carefully placing a target (using a level) at a known distance away (preferably at 100 yards or further). The target is marked with multiple sets of vertical lines. The test is performed on both the elevation and windage turret. The distance to the target is measured using a precision laser (surveying equipment) or a physical tape measure. NEVER with a standard laser range finder (these are often +/- 1 to 3 yards error at 100 yards). You measure from the elevation turret to the target. You then perform the tall target test and use the following document to complete the math: http://appliedballisticsllc.com/Articles/TallTarget.pdf the end result is the Scope Calibration. Now, it isn’t uncommon for different portions of the elevation turret to have different calibrations, so you want to perform this in 3 or more zones of the turret.
Often a scope that doesn’t track absolutely perfect tricks the user into thinking the firing solution has an error in it. You can account for this easily though by adjusting the muzzle velocity, or using a solver with the ability to adjust Sight Scale Factor. Another thing to remember when performing the test is that some scopes are True MOA (1.047 inches at 100 yards) and some are Shooters MOA (1 inch at 100 yards). So make sure when you perform the calibration you keep this mind. If you use a solver that only has MOA & Mils, you can use a calibration on a scope that is Shooters MOA to correct it to True MOA for a ballistic calculator.
Finally – two books that should be in your library….