Home Forums Hunting & Shooting Hunting Optics How to test your scope for proper function/tracking before going to live fire

I test all my new scopes for tracking and RTZ on the bench before I test them with a TTT and live fire. Since it has come up several times recently, I thought I would show a quick and simple way you can test your scopes for proper reticle subtension and tracking without removing them from your rifles. You can use this method without the need for an expensive jig or collimator, and without clamps or a playground handy. You can do this in your back yard or even in your basement. You scope does need a reticle with some type of subtension mark for this method to work, so it can't be used with something like a plain crosshair. I have used this method to test many scopes for tracking and RTZ, and my field results/shooting have corroborated the testing results without exception.

I'll preface this by saying that you will need to invest in a few inexpensive tools that are useful for other purposes as well. Here's what you'll need:

- Bushnell (or another brand) optical bore sighter with grid "reticle". In addition to testing your scope's tracking, I have also found this to be useful for recording zero on my scopes, and getting back on target within a couple of tenths of a MRAD when removing and replacing the scope, or when replacing a scope with another and getting a rough zero on the new scope before firing the first shot. If you trust that your reticle's subtensions are accurate, this is the only tool you'll need, and you can skip the rest of the list. Here's what I use: https://www.amazon.com/Bushnell-740001C-Magnetic-Boresighter/dp/B001C5ZRMI
- Tape measure
- Piece of paper, ruler, and a pencil
- IOTA. This is a flip-up scope cap with a lens inside, used to reduce the focal distance of your scope (to about 12-15') , and it also reduces the parallax at this distance. If you have a scope that can focus and minimize parallax at the distance you'll use to mount your piece of paper to a wall, fence, etc, then you don't need the IOTA. This device is, however, very useful for checking your reticle subtensions, as well as for dry-fire practice when you have limited space. An IOTA can be made or purchased. https://www.65guys.com/dry-fire-practice-and-the-indoor-optical-training-aid-iota/
- A gun cradle or other way of mounting your rifle so it is immobile. A cleaning vise works well for this.

To begin with you'll check the subtension markings of your reticle, as these will be used for tracking testing later. Begin by mounting your rifle in the cradle/vise so it is facing a wall, fence, or something similar that is approximately 12-15' away from the rifle (12-15' is important if using the IOTA, otherwise you can use a longer distance if you have it available, and in fact the longer the distance, the less the relative error in your results). Install the IOTA flip-up cap on the objective housing of the scope. Now take your tape measure and find the distance between the objective lens of the scope and the wall. Measure this distance as precisely as you can. Now determine the subtension increments of your reticle that you want to measure. For the SWFA SS MQ reticle you can check its elevation markings at each 0.5 MRAD increment for 15 MRAD, if you want, but for this demonstration I simply wanted to test the subtension of the 10 MRAD marking. Now you'll need your paper, pencil, ruler, and a tiny bit of math.

If you know that you want to mark a true 10 MRAD on the piece of paper, and the distance you measured between the wall and the objective lens is 15', for example, in order to determine how far apart to make your markings on the paper you use simple trigonometry and the following equation:

tan(angle in radians) = distance between markings on paper / distance between the objective lens and the wall

We can use the small-angle approximation to make this even simpler, and we get

angle = distance between markings on paper / distance between the objective lens and the wall

Now rearranging the equation we see that

distance between markings = angle * distance between the objective lens and the wall

For this example, in which 10 MRAD is equal to 0.010 radians, the equation becomes

distance between markings = 0.010 radians * 15'

Giving 0.15' between markings. Now depending on the units that your ruler uses, you can convert this to cm by multiplying by 30.48, or you can convert to inches. Assuming your ruler is marked in inches, the distance between markings would be 1.8". So make a mark with your pencil, and then another mark 1.8" below the first. Connect the markings with a straight line using your ruler. Try to make sure that the line is square with the edges of the paper.For this example, I printed off a target pattern that I use for load development, and made a mark the correct distance from one of the printed horizontal lines. Now tape the paper to the wall so the line on the paper is aligned with the vertical crosshair on the reticle when looking through the scope.

Position the rifle (or dial your turrets) so that the center of the reticle is aligned with the top mark on your paper, and if your reticle subtension is accurate the 10 MRAD hash mark should line up exactly with the bottom line on your paper. This should look something like this (ignore the upper of the two marks that I made with pencil; I was using that for something else), where the center of reticle is aligned with one of the faint 1 cm lines, and the 10 MRAD hash mark is aligned with my bottom pencil mark. It was difficult to capture a decent image through the scope, and the resolution makes it tough to tell, but the markings on the reticle match up perfectly with the markings on the paper, indicating that the reticle hash mark subtends exactly 10 MRAD.



Now that the subtension of the reticle is confirmed (and you can confirm the subtension of each hash mark individually), we will move to the tracking test. You can do this part with or without the IOTA, but each time you look through the scope you'll need to make sure you center your eye in the image of the scope, to eliminate parallax. The IOTA reduces parallax at close distance so is helpful, but not essential. Your rifle doesn't need to remain in the gun vise for this part, but you'll need to rest the forend on something. Attach the magnetic boresighter to the muzzle of the rifle, ensuring that the rifle is aimed at a location that is well back-lit. Now align the optical grid in the boresighter with the scope's reticle by twisting the boresighter on the muzzle. After the grid and the reticle are aligned, look through the scope again and move the windage on the scope until the vertical line of the scope's reticle is visually "on top" of one of the lines in the grid. Check again to make sure the reticle and the grid are perfectly aligned. The mounted boresighter looks like this:



Now that the set up is complete, check to see if one of the subtension marks you tested before is already aligned with a horizontal line in the grid, and if not, move the elevation turret until a mark is aligned. For this example, I used the 9 MRAD diamond mark on the reticle, and aligned it with the grid line just below the center line, as seen below.



Now, since you have confirmed that your reticle subtensions are accurate (you would have wanted to check the accuracy of the 9 MRAD hash mark, in addition to the 10 MRAD mark that I checked, since that's what we're using for testing the tracking of the erector), we know that if the turrets are also accurate in their adjustment increments, we should be able to dial exactly 9 MRAD into the elevation turret, and the center of the reticle should be exactly at the position that the 9 MRAD mark is at currently. Keep in mind that your eye placement behind the scope should remain consistent and centered on the image of the scope throughout this process, to reduce parallax effects. The position of the reticle after dialing 9MRAD on the turret is seen in the image below:



You can continue to find "witness marks" where the a hash mark on the reticle and a line on the grid intersect, and use that to continue to test turret tracking all the way down until you either run out of grid lines or your erector runs out of elevation travel. As you dial the elevation turret "up", you should see no horizontal drifting of the vertical line in the reticle, relative to the vertical line in the grid that you're aligned with. Dial back down in the other direction to confirm the hash marks line up with the same grid lines, using the same settings on the turret, and then dial back to zero to confirm correct RTZ. The same concepts can be used to test the tracking of the windage turret along a horizontal axis.

If your turret adjustment increments do not match your reticle subtensions (which we have confirmed to be accurate), you can figure out the increments that your turret actually adjusts, and then you can calculate the correction factor to use in your ballistic solver software. If it only takes "8.7 MRAD" dialed on the turret to move the reticle a measured 9 MRAD, then your turret needs to be dialed "0.97 MRAD" for every 1 true MRAD moved by the reticle. If you need to dial "9.2 MRAD" to move the reticle a true 9 MRAD, then your turret gives "1.02 MRAD" dialed for every 1 true MRAD. This is found by dividing the amount dialed by the amount the reticle actually moved. Some ballistic apps use this convention of "correction factor" (like Shooter, IIRC) and some use the amount actually moved for every 1 MRAD dialed (like Ballistic:AE), in which you divide the number moved by the amount dialed. This would produce the inverse numbers to what I wrote above (8.7 dialed for a true 9 MRAD reticle movement would give a correction factor of 1.02, and 9.2 dialed for 9 true would give a factor of 0.97).

So we have confirmed the subtension of the hash marks in the reticle, and using those marks we have confirmed the tracking of our turrets, all before we fire a single shot down range, and all without the benefit of having expensive equipment, clamps, or playgrounds. There are more precise and more sophisticated ways of doing these tests, but this gives the average shooter a way to check the tracking and subtension of his scopes without having to spend a bunch of money. And on weeks like this where the windchill puts the felt temp at about -45 degrees, a guy doesn't even have to leave the house to do these tests, if he doesn't want to

Thanks for taking the time to post this. No small amount of work went into this write up. 👍

Thanks Jordan .

I have a few new scopes I'll try this on today

Sticky!

X2

Thanks, Jordan, for the great write-up!
Well-done.

Good information, thanks for taking the time to share it.

A version of this method has been 100% reliable for me also.

Great write up, Jordan. Should be helpful for lots of guys. Thanks for taking the time.

Nice write up!