Posted by NC Scout | May 20, 2020 | AP Editorial Staff, Guest Authors, Tactical, Weapons | 3 | https://www.americanpartisan.org/2020/05/guest-post-some-semi-random-thoughts-for-the-designated-marksman-by-jt/

This is just a small catalog of things I have come across over the years which may help others. Some I have not seen documented or if so, obscurely. Others are just helpful (I hope) pointers.

1) Humidity – unless shooting very long distance (farther than I would typically shoot my M1A-NM) humidity is not going to be a big factor, Not much more than the coriolis effect on my .308. Ignore FM 23-10 Sniper Training, much of which was either written by someone who never shot long distance precision, or was injecting disinformation intentionally for the “benefit” of our enemies. In particular, this gem:

3-17. EFFECTS OF HUMIDITY

Humidity varies along with the altitude and temperature. The sniper can encounter problems if drastic humidity changes occur in his area of operation. Remember, if humidity goes up, impact goes down; if humidity goes down, impact goes up. As a rule of thumb, a 20-percent change will equal about one minute, affecting the point of impact. The sniper should keep a good sniper data book during training and refer to his own record.

No no no, (other than the very last sentence.) In fact, anybody with a high-school education in chemistry (from 35 years ago anyway) should be throwing the BS flag on this. Water vapor is not dissolved in the atmosphere like salt or sugar does in water and thereby increasing its density. Water vapor is a gas, and for a given temperature and pressure, the atmosphere will hold a certain amount of gaseous water per unit volume. This is an application of the Ideal Gas Law. That given volume at the specific temperature and pressure holds a specific number of molecules. The atmosphere is approximately 78% nitrogen (N2 – molecular weight 28), 21% oxygen (O2 – molecular weight 32) and 1% trace stuff (other things.) Water vapor (H2O) has a molecular weight of 18, significantly less than N2 and O2. Molecular weight is the prime contributor to the air resistance on the bullet’s flight, and as pressure goes up, there are more molecules getting in the way. Back to the water vapor. When humidity goes up, that means that there are more water vapor molecules in the atmosphere (the total possible being dependent on temperature)and for every 100 more water vapor molecules added, then there must be 78 fewer N2 molecules, 21 fewer O2 molecules, and 1 less of the “others” (which actually includes water vapor.) Therefore, there is LESS resistance to the bullets flight as humidity increases. I could go more into this, but I don’t want to re-hash the chemistry classes you should have had. I will just end this section with this:

For a given temperature and pressure, as humidity increases, there is less resistance and bullet impact will raise.

As air temperature rises, the amount of water vapor the atmosphere can hold will increase, but without an addition of water vapor coming from somewhere, the humidity will decrease.

The amount of change in bullet impact is pretty small for most of our weapon platforms.

Try to zero your rifle during your average atmospheric conditions (if you can figure that out) and then ignore humidity from that point on in the event you have to take a shot without conducting a calculated firing solution. Alternatively, zero it on a day with close to 50% humidity. You should always keep track of your temperature and atmospheric pressure (or density-altitude) for the conditions when you sighted-in your rifle for use later when calculating a firing solution when time permits.

2) Logbooks. Logbooks are nice to have for a good rifle and are certainly the best way to determine where your round should hit. Ballistic calculators are theory, but the logbook is practice. The problem with them is they take a LOT of time, work and money. And once it is “complete” you’ve made major progress towards the end of your barrel’s life. A good ballistic calculator helps you shortcut that, but you should always spot-check it to see where the calculator needs correction for your weapon. Remember that the ballistic calculator is always theory, no matter how good the results are. You always need to check its results with real-world test-checks of your weapon’s (and your personal) abilities.

3) MilDot Master. This is a handy tool that makes converting your Mildot reading to range calculations quicker in the field. Similar to a slide rule (more properly termed a slide board) you line up the observed mildot size with the estimated size of the target (or nearby thing) and read the range. It also has an indicated range correction for uphill/downhill angles to get the true horizontal range which is needed for those high elevation shot angles. The system presented for adjusting the hold-under/turret adjustment is an approximation which they admit is of uncertain value, particularly at longer ranges. Also, that adjustment is dependent upon having a ballistic chart present to compare it to. They do give a place on the back of the slide to tape such a chart, but you have to remember that such a chart is only for one particular set of atmospheric or range conditions. The Mildot Master is still a very good product, (one I balked at buying for a long time due to the price) but I do consider it an important thing to have with me. I can do calculations in my head, and pretty fast too, (I worked engineering for several years) but have come to realize that when under pressure I’m still not fast enough, especially when having to adjust for elevation, and longer ranges.

4) While electronic ballistic calculators are pretty snazzy, (I really like Ballistic for iOS – it utilizes the JBM algorithm) my goto calculator is the WhizWheel by Accuracy1st Development Group. (It uses the Applied Ballistics algorithm which gives very comparable results to the JBM algorithm.) No batteries and gives a firing solution faster than your electronic device – seriously. It also includes an adjustment chart for uphill/downhill angles that is pretty good (at least for my load) and is vastly superior to the method suggested with the MilDot Master which is based on John Plaster’s idea (author of Ultimate Sniper.) In addition, like a slide rule (which the Whiz Wheel is actually a specialized version of the circular type,) the Whiz Wheel is a parallel calculator, not a serial one like an electronic calculator. It allows what in engineering is referred to as a “Monte Carlo Analysis.” That is, it allows the user at a glance without any or at most very little adjustment a visualization of how a change in a parameters besides target range will affect the shot – nearly instantly. Your electronic ballistic calculator requires you to re-enter and recalculate to see those changes. Your logbook requires you to flip to the correct page (if you’ve got it) and back and forth to compare. That alone is worth the cost in my opinion. (For those familiar with the slide rule – or terrified at the mention of it – the Whiz Wheel is much simpler to use, while literally being the most sophisticated version of one ever that I heave seen – and I’ve seen A LOT. The standard slide rule can perform a multitude of functions, but the operator does them one at a time. The Whiz Wheel does a multitude of complex functions all at the same time.) (Second Note: A pocket slide rule – 5″ – is a good thing to have with you in the field if you are also a RTO [Radio Telephone Operator] because for the Monte Carlo Analysis reason you can check for multiples of desired/undesired frequencies rapidly.)

5) While both the MilDot Master and the Whizwheel (and most if not all electronic ballistic calculators that may help with the mildot conversions to range that I’ve seen) will give you a method to correct your trajectory for the shot when there is a difference in elevation between you and your target, true point-to-point (straight-line) range is not adjusted for when first *estimating* that range. This will lead to an incorrect initial range estimation. For lesser differences in elevation, it is not noticeable. (e.g., a difference in elevation resulting in an 18 degree firing angle will result in a 5% error in the straight-line range estimation.) This is trigonometry, and the error is always such that you will overestimate the range. This is an effect (similar to the effect “parallax” that is also important to the shooter using scopes) that I just haven’t seen addressed to any meaningful extent. At close to medium distances for your weapon this isn’t necessarily horrible, but at longer and especially extreme ranges when the bullets trajectory is affected by an ever-increasing rate downward (toward Earth) this can be catastrophic. It just so happens that the error in range estimation is inversely proportional to the cosine of the angle between you and the target. It also just so happens that it is the same as the approximation adjustment when adjusting from the true straightline distance to pure horizontal distance when calculating the holdunder for the firing solution (and also for wind-effects correction on the shot for its angle.)

A simple example would be to take a book and stand it up on a table across the room and take note of its apparent height using your hand or fingers.

Now go over and lean it back against something, go across the room to where you were before and using the same hand and finger method note its new apparent size.

This is the exact same effect that you have to compensate for when estimating range.

Because of the bullet’s trajectory (initial rise and then fall) this trigonometric effect is most important when estimating range using vertical measurements with the optical rangefinder whether mildots or another reticular system. If you are sizing based on a horizontal measurement, there is still a trigonometric effect if the object you are using is bladed to you – or not facing you head-on. This situation, however, is more difficult to calculate because you will not be able to make a precise estimation of the angle the object is bladed to you. For this reason, I think it is better to use a vertical measurement when estimating the range.

With vertical object range estimation, it is fairly easy to get a good angle measurement with any one of a number of cheap or easily made devices. The Mildot Master even has one built in. Prior to acquiring that, I used a small plastic protractor with a hole drilled at the origin(center) and used a small weight attached to a line. There are even specially designed levels for attaching to your scope/mount that will give you the angle or cosine value directly.

So, once the angle is known, the apparent range is multiplied by the cosine of the angle to get the true straight-line range. If you have a MilDot Master, that is already provided for you, although the instruction manual that comes with it never addresses that. You simply use the same part of the MilDot Master that is used for converting the straight-line range to horizontal range. In effect, you will be using that adjustment twice: once to correct the apparent range to true straight-line range (due to the object not being at a right angle to your line of sight), and then again to get from straight-line range to horizontal range.

Example: you go out to do some target shooting, and place your target stand on the ground with a 3-foot tall target sitting vertically on the ground, facing a steep hill. You climb the hill (probably from the backside due to the steep incline on the side toward your target) and look down at your target, and it is way down there! Through your mildot scope you see that your 3-foot tall target is exactly 1 mildot high! Oooh-boy! that’s easy! The target must be 1000 yards away! Nope. You pull out your protractor-like device and see that the angle from you to the target is a steep 45 degrees. If you had placed the target so that it was tipped back and facing you directly head-on and it was measuring 1 mildot, then yes, it would be 1000 yards away. (You would still have to adjust for the elevation difference to get the horizontal range, and then calculate the hold-under for that, but at least you wouldn’t have to adjust for the apparent size shift of the target due to it being vertical.) Back to the range estimation. Because the target is angled vertically from you at 45 degrees, you have to multiply the apparent range indicated by the measured size of 1 mildot by the cosine of 45 which is approximately .7 giving you a true straight-line range of about 700 yards. Had you angled the target 45 degrees so it faced you head-on, that target would have measured a little bit over 1.4 mildots in height. As I mentioned, you can use the mildot master to figure this out without doing the calculations by using the same procedure they provide to calculate the true horizontal range. Just remember that the procedure they gave is for use once you already know the correct straight-line range. You have to get to that point first. So if you don’t know that, you will end up having to perform that procedure twice before arriving at the firing solution. Therefore, since we now know that the true straight-line range to our target is 700 yards, we still have to compensate for the bullet’s flight at this angle and so calculate the horizontal range. We’ve already got our cosine for the angle so we now can figure out that our horizontal range is about 490 yards, and it’s that 490 yard range that is used to adjust our hold-under for our firing solution.

Lastly, this adjustment for range estimation (and not the horizontal range adjustment for bullet drop compensation) is not applicable for some types of objects that have “depth” apparent from these angles. For example, a horizontally laid propane tank is a cylinder, that will always have the same vertically apparent height no matter your elevation. Think about a basketball and how it will always appear the same no matter what angle you look at it.

I hope that some of these things helps those who are working at their longer range skills.