Calibrating Mildots on a Mildot reticle

[David.Soliman]

I have seen lots of people trying to use Mildots for range estimation and hold-over with the help of softwares like Chairgun and the likes, with very little or no success at all. Even though they have very accurately entered data like scope height, zero range reticle type, magnification, cal., BC and even wind and atmospheric pressure, still the data produced from Chairgun caould be way off.

In my personal experience this is because the Mildots on the reticle is not actually calibrated to the correct distances, the distance between each dot is (or at least should be) 1 Milliradian. On cheaper/more affordable scopes the calibration is usually off, just like the parallax setting on an AO scope, the numbers don't usually mean anything.

This is how I calibrate my cheap Chinese scopes, and even check more expensive ones. On a variable power scope the magnification wheel would have numbers indicating the magnification, and if you play with Chairgun a little changing the magnification you will appreciate how the magnification can change where the pellet appears to land on your reticle.
For example these are how the poi appears on a Mildot reticle when the power is set at 4X



And this one is for the same gun/pellet/scope/conditions when the scope is set at 16X



For my purpose I couldn't care less about the true magnification all I care about is calibrating the mil dots to make it work with Chairgun.

A bit of maths, this is how the range is estimated:




If we change it around a bit:




If you have a target of a 10 cm size placed at exactly at 10 meters:




We end up with this:




So my aim is to re-calibrate the magnification on the power wheel so that the mil dots represents a true milliradian




Draw a 10 cm line on a sheet of paper and fix the scope at exactly 10 meters from the paper




Set the parallax to the correct setting and check by moving your head from side to side until the reticle stays on the target without shifting.

Set the magnification to the lowest and look through the scope and start counting the dots, the number of dots = Magnification

For example this is 6 X



Write 6 on the power wheel

This is 9 X



Write 9 on the power wheel.

Doing this I found that some expensive scopes are very accurate on the power settings, others are slightly shifted while some cheap Chinese scopes labeled 3-9 are actually 2-5.

I hope this helps some of the new folks out there that might be confused and are asking about using Mildots.

[Josan]

I like your approach. I don't see many people calibrate their mildots. In fact i thought i was the only one.

Here's how i do it:

• Take a piece of paper, ruler and pen
• Suppose you want to shoot at 8x
• put a few dots seperated 4 cm apart on the paper, place the paper at 8 x 4 = 32m The mildots should align with the dots on the paper. If not, turn the magnification untill they do.
• You can do this with any magnification or dot separation.
• Suppose you want to shoot at 12x and love your dots to be 3 cm separated
• put a few dots seperated 3 cm apart on the paper, place the paper at 12 x 3 = 36m The mildots should align with the dots on the paper. If not, turn the magnification untill they do.

Now you have your mildots calibrated, you can measure distance if you know any size in the feild by the formula:
distance in meters = (size in cm x magnification)/size in mildots

Making Chairgun trajectories match reality is a different challenge. Here's what i do.

• Measure the pellet muzzle velocity.
• Enter scope-height from measurement.
• Enter pellet BC from literature.
• Now put a targetpaper at different ranges and measure the drop of the pellet either with a ruler on the paper or take a note of the drop in mildots through the scope. Either one is good.
• Go back to the computer and make the Chairgun curve fit on the short range ( up to plusminus 15m) first by changing the scope height and make the curve fit by changing the BC value for the longer ranges past 30m.
• Now you have a curve that will match the real world pellet-trajectory.
• If you change pellets you can keep the scope height and only have to enter the new muzzle velocity and tweak the BC until the curve fits the trajectory of the new pellet.

There's a reason why this is correct and works.
The scope height determines the POI for the shorter distance but almost not for the farther distances.
The BC value however is almost irrelevant for the pellettrajectory in the first 15m, but determines the pellettrajectory on greater distances to a very large extend.
BC values from the internet are bogus. They vary greatly with muzzle velocity and gun. What you are doing here is empirically determining it's value. Chairgun even has a calculator for this that does exactly the same thing, namely calculate it from drop at different ranges.

[Airgun.Sniper]

 I now tend to use FFP scopes, but back to SFP. On most scopes (hawke, centerpoint, and some leapers (unless straight 10x) that have a mildot are usually set on 10x. That being said Vortex mildot scopes that have mildot reticle have a TRUE mildot are on 12x or 14x. You need to check manufacturers TRUE mildot specs in your owners manual.

  That being said true mildot is at 100 yards 3.6". That is the distance from dot to dot. Now for airgun purposes this is too much. What one can simply do if they understand basic math is break it down into quarters. I have done this just in case people need help. Proportions will help you solve this.
 
 100 yards = 3.6"
   75 yards = 2.7"
   50 yards = 1.8"
   25 yards = 0.9"

  Now if you want it even smaller just divide it up again. For instance

 50 yards = 1.8"
 45 yards = 1.62"
 40 yards = 1.44"
 35 yards = 1.26"
 30 yards = 1.08"
 25 yards = 0.9"
 20 yards = 0.72"

 You can break them down farther then that. Lets use my benjamin mrod in .22. I know at 50 yards I have a - drop of I have 4.96". Now just set 4.96" over 1.8" and I know my hold over is now 2.75 mils.

 Just as simple as that. I do this for all my guns weather it be powder or air. Its not too hard and rather fun.

[Motorhead]

For calibrating our "Hunter Class" field target scopes ......
At 12X ..... ruler out at 100 feet ..... 1 mil dot per inch viewed when exactly on 12x.
Once done using Chair gun or like programs become a lot more accurate.

[sscoyote]

I also like to play with the math behind retice rangefinding and downrange zeroing. Another way to calibrate the magnification vs. subtension is to simply reticle rangefind various target sizes at known distances then "reverse mil" the true subtension at that particular power (the power the mil is supposed to be cald. for). Then you can tweek the magnification higher or lower to obtain the true mil magnification if it doesn't match up. The true mil magnification can also be calculated if one wishes since as the 1st poster suggests magnification is ~inversely proportional to subtension.

[David.Soliman]

It's rather strange that you almost never find anything on calibrating the Mildot magnification, this seems more important than finding the true parallax setting yet you never find anything on the topic of Mildot calibration and tons of information about parallax settings.
Very interesting to hear all your thoughts on this, and there are some very nice methods mentioned here other than the one I use. 

[Geoff]

tagged for when I have time to try this.  thanks for the info everyone

[Airgun.Sniper]

I hope I answered your question

[sscoyote]

Yes--pretty good info guys. Here's a couple Youtubes I did on the adaptation of the mil-ranging formula for reticle rangefinding and downrange zeroing--
Oh I guess you can't post links here. If you search Youtube- sscoyote I have 2 videos there about the abovementioned topic of which there aren't any others about those topics that I've seen.

As David infers above, oftentimes the best application of something is obscured by what it's designed for and not what it can do. The "MR formula" for rangefinding is only a small part of what it can be used for.

[Josan]

I now tend to use FFP scopes, but back to SFP. On most scopes (hawke, centerpoint, and some leapers (unless straight 10x) that have a mildot are usually set on 10x.

That right Yota. My examples are about these scopes, SFP and mildot set at 10x. I use Hawke mildot scopes and they are like this.

Furthermore i forgot to mention. To make the Chairgun curve fit the real pellet trajectory you have to zero your scope and enter the zero-distance in Chairgun.

[sscoyote]

Some aren't though. My Simmons 6.5-20 WC is setup for 14X, the Nikons often use 12X, and I think Bushnell often uses 14. My Centerpoint 4-16"s are set at 10X as noted above, and at 16 are as calculated (10/16 x 3.6=2.25 inch per hundred yds.).

[Airgun.Sniper]

Some aren't though. My Simmons 6.5-20 WC is setup for 14X, the Nikons often use 12X, and I think Bushnell often uses 14. My Centerpoint 4-16"s are set at 10X as noted above, and at 16 are as calculated (10/16 x 3.6=2.25 inch per hundred yds.).

 Thats what I said. Not all are 10x. For instance my  vortex scope (6.5-20x50 viper) TRUE mildot dot is set on 12x. I know other scopes are like this too. I prefer 10x. with my scopes. Not too much mag and not too little. Only problem I have with sfp scopes is the magnification adjustment. How do you know your exactly on 10x? You could be at 10.1 or 10.2 or 9.9 mag. When coming to range estimation with just mildots accuracy is important. That is the reason why I went to FFP. Now I still on sfp, but the click letting me know I am on 10x.

[Josan]

It's a fact that numbers on the AO and magnification rings/turrents are not accurate, especially with the cheaper scopes. However i don't care what the exact magnification is as long as my mildots are calibrated. It's them that i use for distance measurement not the maginfication.  On several Hawke scopes i have a setting of about 7.5 on the magnificationdial is 8 according to the mildot-reticle.

[Airgun.Sniper]

It's a fact that numbers on the AO and magnification rings/turrents are not accurate, especially with the cheaper scopes. However i don't care what the exact magnification is as long as my mildots are calibrated. It's them that i use for distance measurement not the maginfication.  On several Hawke scopes i have a setting of about 7.5 on the magnificationdial is 8 according to the mildot-reticle.

 Im not understanding. To measure distance with the mildot you need to be on the correct magnifaction...or have a very high scope with great glass and a sidewheel thats accurate. My mueller has very great glass way better then anything $350 and under...I use mildots because i can accurately judge distance, know my holdover, and take my shot. Being able to do this very quickly helps.

[Josan]

 Im not understanding. To measure distance with the mildot you need to be on the correct magnifaction...or have a very high scope with great glass and a sidewheel thats accurate.

Hi David, i use the magnification to calibrate the mildots. At 10x 1 mildot is 10cm at 100m. A mildot reticle that is correct at 10x will, in a SFP scope, when zoomed to 8x have 1 mildot cover 12.5cm at 100m, or 2,5cm at 20m, or 5cm at 40m. So if you put dots on a piece of paper at 5cm apart, your mildots should coincide with the dots on the paper. If not, you can turn your magnification ring until they do. Most likely your magnification dial will not be at 8x. For measuring distance it's important that the mildot reticle is calibrated in this way. Who cares that the magnification setting on the dial is close but not exactly what it says? As long as you can use the reticle to measure distance.

[Airgun.Sniper]

 Im not understanding. To measure distance with the mildot you need to be on the correct magnifaction...or have a very high scope with great glass and a sidewheel thats accurate.

Hi David, i use the magnification to calibrate the mildots. At 10x 1 mildot is 10cm at 100m. A mildot reticle that is correct at 10x will, in a SFP scope, when zoomed to 8x have 1 mildot cover 12.5cm at 100m, or 2,5cm at 20m, or 5cm at 40m. So if you put dots on a piece of paper at 5cm apart, your mildots should coincide with the dots on the paper. If not, you can turn your magnification ring until they do. Most likely your magnification dial will not be at 8x. For measuring distance it's important that the mildot reticle is calibrated in this way. Who cares that the magnification setting on the dial is close but not exactly what it says? As long as you can use the reticle to measure distance.

 Ah mate, your making bloody sense now. Thanks for clearin' that up

[redfish]

Will this mil dot calibration work for a non-AO scope?

Also, if I am measuring 10 metres, is it from the front lens of my scope or from the muzzle of my rifle?

[Josan]

Yes,  it will work on a non-AO scope. Measure the distance from the front lens.

The accuracy with which you measure the distance and put the dots on paper, determines how accurate your distance measurement with the mildots can be.

[redfish]

So i set about calibrating the mil dots on my leapers 3-9 x 40 non ao scope, using the 10cm line at 10 meters measured from the front lens. I used a piece of masking tape to write down the magnifications and if i go by my results the scope is actually 4 to 10.

The strange thing though is that the distances between the magnifications I recorded on the tape are not equal. 4x to 5x is about 1/4 inch, but 5 to 6 is almost twice as far apart. This repeats in that 6 to 7 is about 1/4 inch but 7 to 8 are almost half inch apart. Is this normal or did i do something wrong?  I made sure to stabilise the rifle on a bench and slowly increase the magnification.

[David.Soliman]

Yup the distances between the magnification marks on the wheel are not equal, they usually get stacked closer as the magnification increases.