Precision Rifle Matches

I’ve been shooting local precision rifle matches to hone my craft. I completed the second match two weekends ago. If you are near Navasota, Texas, check out TX Precision Matches. They hold matches the second Saturday every month. Shooters need $50 and 100 rounds.

The shooting is challenging, and humbling. The target size is 1-4 MOA, with 2.5 MOA or so being the average. The match features 8 stages. Each stage is 105 seconds, and 8-12 rounds fired. The match typically runs from 0830-1500. The target range is from 350 yards to 1100 yards, with a typical target being at 500 yards. Most shots are from non-prone positions.

TX Precision indexes shooters’ scores off of the leader in your division. The leader gets a 100%, and other shooters get percentages of the leaders score. If the top score in a given division was 60, and a shooter scored a 45, then his score is 75%.

I can’t recommend enough that you go to these local matches to skill build. The shooters are incredibly helpful, and will shepherd first timers through the whole match. Come prepared to be humbled. You probably won’t score well your first few outings. No, it doesn’t mean you suck. It also doesn’t mean that the sport is stupid. It’s a great chance to learn alternate positions, rapid wind calling, and applying the fundamentals under severe time stress.

The divisions are Marksman, Expert, and Pro for bolt guns. Gas guns are a separate division. A look at the scores shows why. I’ve always known from personal experience that bolt guns are more accurate than gas guns, but the low gas gun scores surprised me.

Carbine Drop Charts

Here I will post hold over/under charts for common carbine calibers. The point here is so that users can stretch the range on SBRs, submachine guns, pistols, and carbines. For more information on muzzle velocities out of various barrel lengths, check out Ballistics By The Inch. Of course, I modeled the trajectories in Sendit Ballistics.

9mm, 16″ Barrel

This chart is for a 124gr 9mm fired out of a 16″ barrel. The zero is 25, with re-zero at 80 meters.

5.56mm, XM193, 16″ Barrel

This chart is for a 55gr 5.56 fired out of a 16″ barrel. I used a 30 meter zero, because it re-zeros at 300 meters.

Sendit Ballistics

Check out Sendit Ballistics for iOS if you are interested in an accurate, simple bullet trajectory calculator. I keep an updated page on the app here. The app is so accurate because it models the most important 3 degrees of freedom with painstaking detail. The app is designed for the field user, with a simple interface, range card mode, low light mode, and rapid corrections with minimal information. It is free to 400 meters, and only $4.99 to infinity.

Windage Field Constants for Common Rifle Calibers

For the benefit of the rifle shooting community, I have created this table of common calibers’ wind constants. If you’d like to see another bullet on this table, just post the request in the comment section, and I will update the table (weight, G7 BC, and MV would be helpful). I have gotten it kicked off with 6 common calibers. I made this table using my highly accurate ballistics calculator, Sendit Ballistics. The table is right below. Further explanation is below the table.

You better have some way of getting on paper in this atmospheric shit-storm.

Corrections Table

CaliberRanges Valid To (m)Wind Constant (mil, meter, mph)Wind Constant (moa, yard, mph)
5.56mm, M193, 55gr FMJ, 600206.4
5.56mm, M262, 77 gr BTHP, 2740fps MV800278.6
5.56mm, M855, 62grn FMJ600237.3
6.5mm (CM), 140grn ELD-M10004514.3
7.62mm (300WM), Mk248Mod0, 190gr BTHP11004414
7.62mm, M118LR, 175gr BTHP, 2600fps MV8004012.7
.22 LR, 40gr CCI Mini-mag, rifle40082.5

Having a windage field constant in your back pocket is a great thing for serious shooters. A field constant means a single number which helps you come up with corrections based on a full value wind, and the given range.

The concept of a wind constant works on the following equation:

Meter Line * Windspeed / Constant = Correction

EXAMPLE: 5.56 62grn at 400 meters (meter line 4),
in a 6mph wind. Want MOA correction.

1. Constant = 7
2. Correction = 4 * 6 / 7 = 24/7 = 3.5MOA
  • Meter Line is in hundreds, i.e. the Meter Line for 500m is “5”
  • Windspeed is full value, interpolate for half values
  • Constant is in such a unit that it gives you the desired correction

I made the table with two types of constants. The first row is for shooters who desire a mil correction, with meter distances, and mile per hour winds. The second column shows MOA corrections, for shooters shooting in yards, and using mile per hour winds.

You will notice that some of these constants are non-rounded numbers. I decided to err on the side of giving the rifleman the most information. If you don’t like “27,” you can make the call to round it to “25.”

Methodology

The wind constant calculations are pretty much pure analytical science. I used the ballistics engine in Sendit Ballistics to produce the corrections for the given bullet and wind parameters. From there, it is simple algebra to calculate the wind constant (see the simple equation at the top of the page, we need to know 3 numbers to find the 4th). I modeled the wind constant at every hundred meters, and at 5, 10, and 20mph for each cartridge.

Past those calculations, there is some art involved. I first average all the wind constants to get a baseline number. This number is typically too low. The wind constants change drastically from 100m to 1000m. The constants are higher at shorter ranges, meaning that ultimately they produce smaller corrections for the riflemen in the field. Of course, the small wind constants at long range produce corrections which are too big at medium ranges. The key is to pick a reasonable range for the given cartridge, and then select a wind constant that will give adequate coverage for both medium and long ranges for that cartridge. It’s not too much of a problem if the shooter overcorrects by a large amount at shorter ranges; it amounts to centimeters.

Sendit Ballistics

Check out Sendit Ballistics for iOS if you are interested in an accurate, simple bullet trajectory calculator. I keep an updated page on the app here. The app is so accurate because it models the most important 3 degrees of freedom with painstaking detail. The app is designed for the field user, with a simple interface, range card mode, low light mode, and rapid corrections with minimal information. It is free to 400 meters, and only $4.99 to infinity.

Terminal Ballistics from The Weapon Blog

Weapon Blog has posted some excellent articles lately on terminal ballistics. Really, they are videos of people shooting things. Is there any other way to conduct terminal ballistics modeling? In terminal ballistics, empirical data rules over analytical solutions.

In the first video, the shooter fires multiple calibers at a pine tree to determine which ones will penetrate.

In the second video, the shooter fires 5.56, 9mm, and 22LR through multiple layers of drywall.


AR-15 Wind Corrections Made Easy

The AR-15 series is a Rifleman’s rifle. It is accurate, customizable, and easily maintained. Even with off-the-shelf Full Metal Jacket (FMJ) ammunition, it’s a 3-MOA gun. In other words, a 5.56mm can hit a man-sized target at 600 meters without any customization or match ammunition. As many users have verified, upgraded stocks, optics, and ammunition can extend this range. This accuracy holds during good weather.

During wind, the burden of accuracy shifts from the AR-15 (accurate 3MOA or better), to the shooter or spotter who is calling the wind. Out to 100 meters, the wind would need to blow at roughly 60mph to move a bullet off of a man-sized target. Even at 300 meters, the shooter can get away with holding on the left edge or the right edge to compensate for any wind under 15mph.

At 400 meters and further, incorrect wind calls will lead to misses. A mere left or right edge hold limits the shooter to a 7.5mph wind. In other words, the AR-15 shooter’s lack of wind calling skill limits the platform’s effective range to 400 meters on a significant number of days.

The Rule of 7 for 5.56

The Rule of 7 for 5.56 is simple. To get your windage hold, multiply the meter-line and the windspeed in mph, and divide by constant 7.

Example 1:

500 meters, 4mph wind, full value. Need MOA correction.

MOA = 5 * 4 / 7 = 2.85.  Round to 2.75 or 3.0 MOA

Now, you need to know the value of the wind. Your gun is pointed at the 12 o’clock. 12 and 6 are no value. 3 and 9 are 100% value. 11, 1, 5, 7 are 50% value. 2, 4, 8, 10 are 70% value.

Example 2:

620 meters, 8mph wind out of the 1’oclock. Need MOA correction.

MOA = 6 * 8 * 0.5 / 7 = 3.43. Round to 3.5 MOA

Perhaps you don’t have a scope or reticle that works in MOA. You want inches so that you can hold off of the target into space. Remember, shoulder-to-shoulder is 19″ on an average male. Simply multiple the MOA correction with the meter-line.

Example 3:

400 meters, 16mph wind out of the 4’oclock. Need inches.

MOA = 4 * 16 * 0.7 / 7 = 6.4. Round to 6.5

Inches = 6.5 * 4 = 26 inches of hold.

Accuracy Check with Sendit Ballistics

We’ll use Sendit Ballistics, my iOS app, to check the accuracy of the rule, assuming 62-grain 5.56 at 2970fps. Wind is full value 10mph.

200 meters: 3 MOA by Rule of 7, 2 MOA by Sendit Ballistics

400 meters: 6 MOA by Rule of 7, 4.5 MOA by Sendit Ballistics

600 meters: 8.5 MOA by Rule of 7, 8.0 MOA by Sendit Ballistics

800 meters: 11 MOA by Rule of 7, 12.0 MOA by Sendit Ballistics

 

In other words, keep the Rule of 7 in your back pocket for windy days! And keep Sendit Ballistics in your back pocket too, in case you need perfection.

The 2 Common 5.56mm Zero Schemes

Praxis made a useful comment on my 5.56 holdover infographic:

Recognize that the point of impact for all of these aim points is the 300 dot high center chest. So with a 25/300 zero if you take an aimed head shot at 100-200 you will miss high. The 25/300 zero is an infantry zero that trades an area hit probability gain and gives up near range precision. You can run trajectories through JBM ballistics online. MV, sight height, and BC are the most relevant inputs. For most realistic and effective engagement ranges for civies and police an approximate 50/200 zero is more practical.

It’s a good point, and it stands. The long and short of it is that soldiers with battle rifles should use the 25/300 zero, while CQM carbine shooters should use the 50/200 zero.

The 50/200 is flatter shooting. The 25/300 zero has a max ordinate (the highest point in the trajectory) of 6.69″ at 175m, with the M855 round. The 50/200 has a max ordinate of 1.99″ at 123m. Essentially, the 50/200 zero is point of aim, point of impact from 0-230m. Your drop is 5 feet at 500 meters, so this zero is very unsuitable for medium range targets.

GIGO: Why Your Ballistic Calculator is Inaccurate

Sendit Ballistics, my own calculator, has a great reputation for accuracy. My algorithm takes a lot into consideration, and this results in excellent performance for medium to long range shots. However, the truth is that most all ballistic calculators will give reasonably good data.

While calculating trajectories in compressible flow (bullets flying quickly through air) requires advanced mathematics, it’s a fairly mature field. The reason your calculator gives you bad data probably has nothing to do with the calculator’s algorithm. Rather, it is a phenomenon known as GIGO: Garbage In, Garbage Out.

Your ballistic calculator relies on the following data, in order from the most important to least important:

  1. Ballistic Coefficient – This is generally a “G1” or “G7” class coefficient. All it means is that the bullet is benchmarked against a standard shaped object, and its performance is measured relative to that well known drag profile. You can get real nonsense if you input a G1 BC in place of a  G7 BC.
  2. Muzzle Velocity – Get this too wrong, and your range card will be useless past 500 meters.
  3. Weight – Inputing the wrong weight will make your bullet seem to perform better if it is too heavy, or worse if it is too light.
  4. Zero range – If you input the wrong zero range, by a large margin, you can easily get unhelpful corrections.

Less important: scope height, weather, twist rate, Coriolis effect.

Happy hunting.