I have now read three separate articles on the Mil Dot, minute of angle, and the non-existent difference between Army and Marine Mils.
First what is a mil? It is a shorten name for a milradian. That is it is 1/1000th of a radian. So, what is a radian? "A radian is an angular measurement that is equal to the angle formed at the center of a circle by two radii cutting off an arc whose length is equal to the radius." That is a direct quote from the Webster's New World Edition Dictionary. Look at it as a slice of pie in which the "outer rounded side" is equal to the two straight sides. The angle at the pointy end is one radian. How do you find this great angle? OK, radian = 2 pi r/r is the standard formula. So a circle with a five inch radius becomes (2 * 3.1416 * 5)/5 = 6.2832 radians to a circle of 360 degrees. By dividing 360 degrees by 6.2832, you find that there are 57.2956 degrees in a radian. You also know that there are 6283.2 milradian in a circle. Either of these can give you the number of minutes of angle in a milradian or mil. That magic number is 3.438, which is commonly rounded to 3.44. This is a mil is a mil is a mil. There is no difference in Marine or Army mils. The problem is the military compass.
The military compass is marked to show 6400 mils in a circle. The reason behind this I do not know for sure but have been told by numerous sources that is was easier to mark off compasses in 6400 than in 6283. This is also the source of the myth that the Russians use different mils then the US. Their compasses are marked with a different number of mils in a circle. However, again a mil is a mil. It is based on a set mathematical formula that was used by the scope manufacturers in their marking of the reticles. Leupold marked their mils as mils. There are 3.44 moa in a mil. There are no Army and Marine mils. There are only mils.
There are differences in the mil dots however. The Marine mil dot is stamped on wire and the dot is 1/4 mil length or longwise. The Army dot is etched on glass and is 3/4 of a minute of angle or .22 mil. While the Marine dot can be easily broken down into 1/8th increments the Army dot can be easily broken down into 1/10 increments. This is very accurate and breaking the mils smaller then 1/8 or 1/10, when ranging with mils, is asking for a disaster and a miss at longer ranges. The mils are easily broken down to 1/4 mil increments for leads, wind calls, and holds for elevation or missed shot correction. None of our students have had any real problems with misidentification of 1/4 and 3/4 mil increments. It is called training.
While I do not know how all of the scope manufacturers make their mil dots, I do know that all of the scope manufacturers do know what a mil equals. I would be very cautious with the table shown on one of the articles purporting that some are "Army" dots and others are "Marine" dots. The biggest problem in the Mil Dot arena is the introduction of myth verses reality. Such as the great 3/4 mil, mil dot of John Plaster.
Using mils to judge distance is very easy and mils are very flexible for this purpose. Now the mil formula works by dividing the size of the target in millimeters by the apparent size of the target in mils. This is why the 1000 formula works so well. Size of the target in meters times 1000 divided by the apparent size in mils. 1000 is the number of millimeters in a meter. Thus if you have a one meter target and multiply it by 1000 you have converted the 1 meter into 1000 millimeters. Now the neat thing about the mil formula is that it assumes that everything is meters and dutifully gives answers in all manner of sizes. Thus if you were to say the size in yards then the answer would be in yards, if in feet the answer in feet, inches answer inches. Thus a 2 yard tall target milled at 4 mils would become 2 times 1000 divided by 4 which equals 500 yards distance. A 6 foot target milled at 4 mils becomes 6 times 1000 divided by 4 equals 1500 feet which also seems to be 500 yards. I could continue in inches but my head is already hurting. This is great for those easy to do times 1000 deals but car tires, tank fenders and a few others may be a bit more problematic. 45 inches does not do well times 1000 converted into meters or yards. You can convert 45 inches into meters but why not simply multiply 45 times 25.4 (number of millimeters in an inch) and divide that by the apparent size of the target in mils. So now 45 times 25.4 divided by 4 mils equals 285.75 meters. For those of you that wish to work in inches you can use 27.7 instead of 25.4. This tricks the formula into converting the range into yards for you. While this is not perfect, it works. Example is the two yard tall target, 72 inches times 27.7 divided by 4 equals 498.6 meters versus 500 yards using the 1000 formula. I can live with the error. When using the mils you must be able to break the mils into 10ths. This can be done based on the dots themselves. The dots are .22 mil but half is .11 mil. Using this information you then rest the target on the top or bottom of a mil and then measure up. Say, top of one mil to bottom of second mil and the reading is 1.8 mils. Another example would be bottom of one mil and top of second mil and you have 2.2 mils. Combinations of the above will give you anywhere in the tenth scale.
The standard for milling the human body is the crotch to top of head in conjunction with the shoulder to shoulder measurement. Using smaller than this and be accurate, and you will be in the 6 inch point blank zero range anyway.
Mils can also be used to hold for winds and it is taught that way in SOTIC. Dialing on winds can be a disaster when you have only seconds to engage in changing wind situations. The mils will hold at 1/4 mil increments easily which is just smaller then 1 moa or .86 moa. This will get you a hit and if you were real anal, then a light 1/2 mil would be 3/8th of a mil or .43 moa. To me this is just too anal. Wind calls are taken from the center of the target as a standard reference point.
The Mil can be used very successfully in holds for elevation. With the criteria of bullet path based on a 500 meter (or yard) zero, the sniper can place 500 on his scope and hold for any target from push the barrel through his guts to 700 meters. We use this info for our students and give them a standard chart that contains that info. That info is only good for the M118 SB and the M118LR. However, anyone can reconstruct one for his or her particular bullet. You are merely compensating for bullet path within the scope. As an example the shooter would hold 5 mils low for a 100-meter shot with 500 meters on his weapon. He would also hold 1 mil high for a 600-meter shot with 500 on the weapon. The usual problem is that the shooter must hang the target in space for long shots while holding for elevation AND wind.
The mil can also be used for engaging moving targets and here the best practice is to use the leading edge for a trigger point. This is due to the problem of shooters looking at the target to find the center of the target. We have our students use the leading edge to stop this problem. There are three general methods of target engagement with moving target. One is ambushing or trapping. We use this method to teach new students moving targets. Another is tracking and is similar to the quarterback pass. And the third is a combination method. In any of these methods the leads are called in mils based on the leading edge of the target. Engaging movers past 400 meters becomes a pain due to wind considerations. Again mil dots will help. As an example, say the lead for the moving target is 2 mils and there is a 3/4 mil wind. Now if the target is moving into the wind then the lead would be two mils PLUS the wind of 3/4 mils so the observer would give a lead of 2 and 3/4 mils. However if the mover is moving with the wind then the wind is subtracted and the lead becomes 1 and 1/4 mils. Fun huh!
Well this is a quick down and dirty and hope that it clarifies some of the info put out in some of the other articles floating around.
Now that you've read this, click here to see a 16-slide show explaining the above. (Unfortunately this works with Internet Explorer only.)