How to Optically Check a Rifle Scopeby Linden B. (Lindy) Sisk
Revised January 3, 2008
Calibrating Reticle Clicks
Get a yardstick - I prefer to use a 4-foot carpenter's ruler. For visibility, place 1 inch Shoot'N'C dots every inch, carefully, and accurately, on the stick. On mine, I placed two dots side by side every 5 inches, so I could quickly count the dots.
Place the stick vertically positioned 100 yards away, carefully measured. A laser rangefinder may not be accurate enough - a series of careful measurements with a long steel tape is better. Use a plumb line or a long carpenter’s level to ensure that the ruler is vertical.
Place the rifle in a good hold on sandbags or other rest. With your hundred-yard zero on the rifle, aim your center crosshairs carefully at your highest magnification at the top of the stick. Have an assistant crank on 36 MOA, being careful not to move the rifle - it's very difficult to accurately do this by yourself.
With each click, the reticle will move a bit down toward the bottom of the stick.
Note where on the stick in inches the center crosshairs rest when he is done. Repeat this several times as necessary, to get an accurate value.
If your scope is calibrated in true MOA, your reticle should move about 37.75 inches during this process. If calibrated in "shooters" MOA, which is more properly termed Inches Per Hundred Yards, it should move 36 inches.
Whatever the figure you get is, you will now have a scale factor to apply for that scope which will tell you how much each click moves the reticle at 100 yards - and, of course, that will scale proportionally at longer distances.
You can also use this procedure to check a scope which adjusts in 0.1 milliradian clicks – 10.0 mils should cover 36 inches at 100 yards.
Bonus: if you have a good 100 yard zero on your scope for your load, this procedure will also tell you if your scope has enough elevation to get to 1000 yards or longer distances - if the reticle stops moving before getting to your predicted dope for a given distance, you're out of luck.
And the ruler can also be used to check the spacing on the reticle - mils should be 3.6 inches apart at 100 yards. Ten mils should be 36 inches.
Having the scope apparently level to the rifle is necessary to proceed to checking the erector system.
Also, ensure that both reticle axes are at 90 degrees to each other. If they aren't, you'll have trouble telling when the scope is level, as it will appear level if you're looking at one axis, but not the other.
You'll need a steady rest for the rifle, which is why you'll probably need an assistant, as it's difficult to hold the rifle and crank the elevation knob.
With your hundred yard zero on the scope, you need to aim the scope at the top of the ruler on one side of it, with the reticle level, and have the assistant crank on elevation. The scope should track toward the bottom of the ruler, staying right along the side of the ruler which you started on.
If there is a deviation from the that vertical line, it means one of two things: either the reticle actually isn’t level, or the erector system wasn't installed so that the scope tracks vertically.
If you get a deviation, I'd make small adjustments by rotating the scope in the rings slightly, and see if you can correct the deviation.
If the reticle then looks like it's off level, you have a problem, because you will most likely get the reticle visually level before shooting, and at long distances, that can materially affect your accuracy.
I have most often used this method to fine-tune the reticle orientation itself. It's rare to find a scope in which the erector system actually doesn't track vertically.
If you do this test, though, you can be sure that your reticle really is vertical.
This procedure may seem complex. However, it has the advantage that it can be done anywhere you have 100 yards of open space, as it doesn’t require shooting, and it eliminates the uncertainty of shooting a group at different elevations which can result from trying to determine which point in the group to use. In addition, it can be done repeatedly until you are certain that you have an accurate result.
Using Second Focal-Plane Scopes at Half Power
Some scopes come with reticles graduated in milliradians, MOA, or Inches per Hundred Yards placed in the second focal plane.
How do you know if it's in the second focal plane? If the reticle doesn't change size as you adjust the scope power, it's in the second focal plane.
Those reticles can be accurately used for holdovers, holdunders, wind holds, or moving target leads only at one power, which frequently is the highest power in the zoom range.
That range may be too high to have a useful field of view for certain types of shooting. An example is the Leupold 6.5-20X50M1 with a mildot reticle placed in the 2nd focal plane. Ten power is a much more useful power for shooting multiple target engagements and moving targets at close range.
Below is a procedure that can be used to calibrate the power ring so that the reticle can be accurately used at half of the maximum power. This procedure is described for mil-based reticles – but with changes in the dimensions, can be used for any reticle.
The Leupold scope described above has ten mils between the thick reticle marks at the top and bottom of the reticle mildots. Ten mils at a range of 100 yards subtends an arc which measures 36 inches or three feet – because one mil is 3.6 inches.
So, take a 7 foot piece of lumber, and accurately place three orange Shoot’N’C dots or other highly visible marks 3 feet apart, so that the top mark and the bottom mark are now 6 feet apart with one in the middle exactly between them.
Place that vertically exactly 100 yards away.
With the reticle at full power, check to see that the top and bottom thick reticle marks fall on the dots 3 feet apart, either the top set or the bottom set, which are 10 mils apart.
If they don’t, then the reticle spacing at full power is incorrect. If that’s the case, you might wish to see if you can dial a power where they are exactly 10 mils apart – and, if so, make a new mark on the power ring so you’ll know where it is when you want to use the scope for range calculations using the reticle at full power.
Now, carefully dial the scope power down – which enlarges the field of view of the scope – until the top and bottom thick reticle marks now exactly subtend the top and bottom marks on the stick. That is the point at which the scope is at half power. Make a durable mark on the power ring so you can return the scope to the power with precision.
Do not be surprised if the half-power mark is not close to the manufacturer’s mark which would correspond to this power. One Leupold 8.5-25 I performed this procedure on recently had the half-power point exactly on the 2 of the mark which indicated 12X – so it was off by 2. If you follow the procedure carefully, you can trust your measurement a lot more than you can trust the manufacturer’s marks.
Now, since the field of view is twice what it should be, to use the reticle at this new power, take holdovers, holdunders, wind holds, and moving target leads calculated in mils, and divide them by 2, i.e., cut them in half.
Think of it this way to remember that: the field of view now covers twice what it used to, so a mark which accurately covered 36 inches at full power now covers 72 inches.
If, for example, your ballistic program calculates that the hold for a target moving 2.3 miles per hour at 400 yards is 1.5 mils with your load, then you will use half of that, or 0.75 mils, with the scope dialed down to half power.
Similarly, if you would use a holdunder at 200 yards with a 400 yard zero of 1.75 mils, with the scope dialed down to half power, you will use about 0.8 mils instead.
This procedure can be used with a scope reticle graduated in any units, simply by changing the measurements to an appropriate unit.
This file is: http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/ScopeChecking/
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