Thread: Checking Scope Adjustment Accuracy Without Firing A Bullet

I would like to share with you guys a method to check a scope's adjustment's accuracy without firing a single bullet. I figured this one by myself (not that hard anyway), but if someone else had already figured it out in the past then good on ya.

The windage and elevation adjustments on scopes are usually marked 1/4 moa, 1/8 moa, or 1/10 mil per click. However that does not mean the scope will actually adjust that much per click. Even high end scopes are prone to having some inaccuracy. The inaccuracy can manifest in three ways. First is the clicks do not actually adjust as the value as labelled, they adjust a little more or a little less. The second is when you make one adjustment, say elevation, there is a tiny bit of movement occuring for the other adjustment. When you make 5 MOA elevation adjustments you could end up getting 0.5 MOA windage change to the right, for example. The third is a combination of the first two.

You can go to the range, fire several shots to do the same adjustment accuracy check, but that is time consuming and cost you ammo. Further, if you got a gun that is not super accurate then you will have a hard time telling how much does the gun's own inaccuracy have to do with the result you are getting.

This method can be perform in most people's own backyard, and it does not cost any bullets.

For the tools you need:
1. a cardboard box with a plain, unmarked side, more than 30cm long/tall, and can stand up.
2. a laser sight, a 50 dollar cheapy is good enough, but it must have the on/off switch connected to the sight body using a rubber wired. In other words, there is no torching of the laser sight when you turn it on and off.
3. a double or reverse scope mount that allows you to mount the laser sight sturdily onto the scope. You can buy them very cheaply on aliexpress. about 3~7 bucks postage included.
4. 25 metres of open area, 50 and 100 are better. Make sure you have no one else around, especially no children or pets.
5. a sturdy tripod (optional).
6. a piece of wood that you can mount a scope rail, and then mount the wood onto the tripod (optional).

now, this is what you have to do to set it up.
1. draw several lines on the cardboard box, 4 lines vertical and 4 horizontal. Mark each cm. Each two lines should be 5 cm a part. You should end up with 9 x 5 cm squares. with a total of 16 line crossings.
2. place the box 25 metres in front of you, make sure the horizontal lines are level. you can check it by holding a piece of string with some weight tied to the end, see if the string line up with your vertical lines.
3. turn you scope to roughly the starting point of the elevation adjustment range, but not to the absolute end (because there adjustments are more likely to not work accurately). about the middle of windage range.
4. mount the laser sight sturdily onto your scope .
5. adjust your laser sight so that the laser dot points to the same point as the centre of your scope crosshair. You must then turn off the last because when you fiddle with the scope adjustment the laser is best turned off to prevent accidental eye damage.
6. mount the scope onto the wood and then onto the tripod. you can alternatively leave the scope on your rifle and put the rifle on a rest or a bipod.

how to use this set up:
1. First, you need to understand the math. Say your scope is 1/4 moa per click, it would mean it takes 4 clicks to shift the poi by 1 inch at 100 yards, or 2.91cm at 100mm, or 0.73cm at 25m. Or, 0.182 cm per click at 25m. To shift POI by 10 cm, you will need 55 clicks.
2. now turn your scope by 55 clicks up. then, point the crosshair at the bottom left most line crossing (the bottom left corner of the bottom left square).
3. now turn on the laser. If your scope has perfectly accurate elevation adjustments, the laser dot should be pointing at the line crossing 2 squares above, exactly 10cm above. If the laser dot is not on the crossing, but is on the horizontal line 10 cm above, then it means your scope moves horizontally when making elevation adjustment. If the laser is not on the crossing, but is on the vertical line, then it means each click is not 1/4 moa.
4. if elevation was not perfect. note how far off is the laser from the crossing. you should be able to work out the rough percentage of how much it is off. say for example, if it is 1cm to the left, then it is 10% off to the left. if it is 0.7cm lower, then it is 7% short (of the adjustment value).
5. try turn a further 27 clicks up, note how much the laser is off the third crossing up. then turn back down 55 clicks (and it would be 29 clicks from the starting point), again note how much the laser is off from the first crossing up., and finally turn back down another 27 clicks and note if you are back to the same spot as the laser.
6. if your scope is off only vertically, but is consistent and can go back to zero, then it is actually pretty good. You can just make a note of how much off it is and take that into account when you shoot in the field and make elevation adjustments.
7. if your scope is off only horizontally while making elevation adjustment like we just did, but is consistent and can go back to zero, Then you need to do the same test for windage and see if it has perfect windage adjustment. I think you can live with 1 crooked adjustment but if both elevation and windage are crooked then the scope's adjustments will be too difficult to calculate.
8. if your scope is off both vertically and horizontally while only making elevation adjustments, you can give up now, the adjustments are impossible to rely on.
9, if 8 above does not apply to you, go on test and examine your scope's windage adjustment like we did from step 2 to 7.


The tripod is not necessary but it does make it easier to hold the scope still and make a careful reading of where the laser dot is.

When I feel like some more math challenge, I will design the squares on my computer so that when it is printed on a piece of A4 paper it is exactly accurate to scale.