Precision Shooting 1-Part 2: Optics, Data and Logic
Now, this is an easy adjustment; a simple equation that will put you very close to right on target. However there are three ways to obtain this and each one is more accurate than the other. The caveat here is that different calibers react differently to the models of correction.
The first method is called the “field expedient” or “Rifleman Method.” As an example, when a surveyor is shooting a mountain top for mapping purposes, he uses an instrument called a theodolite. The theodolite tells the surveyor the angle of his aim. His/her goal is to obtain the base of the triangle distance. Hence, simple geometry comes into play. The surveyor notes the angle that he is holding at, then goes to his data book and obtains a cosine number of that angle, which he then multiplies to the sloped distance.
In figure 2, you can see the sloped distance to target equals 500 yards and the angle that the hunter is holding on is 30 degrees (cosine number of .87). To obtain the bottom leg of the triangle, you would multiply the cosine number of .87 to the 500 yards. (.87 X 500 yds. = 435 yds.) This gives you the straight line distance as if you were shooting on a flat plane, with the full force of gravity affecting the bullets path of flight. However, the”Rifleman Method” does not take into account the fact that the bullet is still traveling the full distance to target, retaining it’s time of flight. Because of this, the Rifleman Method is not adequate for long distance shooting, however the other methods are; and we will cover them shortly.
Below in figure 3, referencing a .300 Remington Ultra Mag, utilizing a flat shooting 180 grain Nosler Partition bullet, with a velocity of 3250 feet per second, the uncorrected for gravity distance of 500 yards to target would cause the bullet’s point of impact (under the field expedient method) to hit approximately 13.2 inches high; or 1.75 minute of angle. Please see the Exbal Software Generated Angle Drop chart below in figure 3.
To the average hunter, these trajectories, at first, may not appear to be significant, but as an experienced hunter you know that they are; and the angle that you will be holding on demands a correction. As an example, let’s say that your rifle shoots an average group size of one inch at 100 yards (large for a pro-grade rifle). At three hundred yards the group size will open to three inches; at five hundred yards, five inches. So, if you are going to be thirteen inches high at 500 yards if uncorrected for gravity, then you can add an additional five inches to that, theoretically, in any direction. This is not difficult to see; math is math and science is science and when the math is done correctly, one round will put one animal down.
Figure 3 – Angle Drop Table for the 300 Remington Ultra Mag
The U.S. Military as well as other Government agencies, train all of their Precision Marksmen on how to obtain the corrected for gravity distance to target. The original method of approach was to utilize a Protractor, string and paper-clip. The string was tied to the center of the protractor and weighted with the paper-clip. When the Marksman was aiming at his target, the protractor was held in place with the weighted string along the side of the receiver inline with the barrel. The Marksmen would then carefully grab the protractor and string, and obtain the angle that he was holding on. Then, the Marksman would go to his data book and obtain the cosine number, then do the math; very simple and very straight forward. Only today there is an easier, faster and more dependable method of obtaining the cosine number; and that is by using an “Angle Cosine Indicator” manufactured by “Sniper Tools Design Company.”
The Angle Cosine Indicator, (ACI) pictured in figure 4, is a widely excepted method of obtaining the cosine number of the angle that the hunter is holding on, by all Branches of the U.S. Military and militaries throughout the world.
It is a simple tool for hunters who hunt in mountainous terrain and is a vault solid precision instrument. It is manufactured from aircraft grade aluminum and anodized a flat black color. It fastens onto your rifle or your scope; either by a standard Weaver Base scope ring, or as seen in figure 4, by “Badger Ordnance’s” military specific picantinny rail mount. When the rifle is held on target, the “ACI” indicates the cosine number of that angle by means of a highly visible index mark; in addition, the cosine numbers transverse the body in five degree increments. The ACI is easily zeroed to your rifles bore by simply loosening the side screw and rotating the body until the zero cosine number sits inline of the index mark.
To install the Angle Cosine Indicator you will first need to decide on your method of mounting; either a Weaver base scope ring or a Badger Ordnance's Picantinny rail mount. Once that is decided, you will level the bore of your barrel by placing a spirit bubble level on the inside rail of your receiver, which is where your bolt lugs ride on. Once the bore is level, install and zero the ACI, insuring that it is indeed level with the bore of the barrel and the zero cosine number is sitting ontop of the index mark. Once that is accomplished, you are ready to hunt. The following is the procedure for utilizing the ACI while in the field.
As previously discussed; with the Rifleman Method, you spot your target. 2) Range / obtain the distance to your target by either utilizing a laser range finder or a ranging reticle. 3) Aim at your target and then look off to the side of your rifle at the Angle Cosine Indicator and obtain the indicated Cosine number. 4) Multiply the cosine number to your distance to target, which will give you you're straight line distance (bottom leg of the triangle) or estimated, corrected for gravity distance. Now, look at your data card to obtain your hold for the corrected target distance, and adjust your turrets as specified.
In the second method of obtaining the corrected for gravity distance to target, known as the “Improved Rifleman Method,” the ACI still plays its role, however the cosine number is multiplied to your minute of angle (MOA) or mil-radian (MILRAD) hold for the specified distance. This is more accurate than the Rifleman method.
*In the third method of obtaining the corrected for gravity distance to target, the ACI again plays its role, however the indicated cosine number is inputted into a Ballistic Targeting Software Program, which delivers the scientifically correct hold. "Exbal" is written by Gerald Perry of "Perry-Systems," and “Field Firing Solutions”, is written by Blaine Fields of “Lex-Talus Corporation. Both are ported to run on three different platforms; 1) Mobile Windows based Pocket PC; 2) Windows PC desk top and 3) Palm Pilot PDA. The Windows based Pocket PC such as the Dell Axim 30 (624 MHz, and probably obsolete by now) is what I use and strongly recommend. This is the most accurate method of use available because the software takes into account the trigonometry of the fact that the bullet will still travel the full distance to target, maintaining a similar time of flight and includes the bullets ballistic coefficient and unique deceleration curve into the equation. If you experiment with your ballistic software, you will find that the Rifleman Method can be as much as eight (8) minutes of angle off of target, whereas when utilizing the Ballistic software, you will be as on target as possible.
Back to the range card. The following example of the range card is pretty much filled in. The “holds” are in MIL marks, but could also be displayed in “minute of angle” or “clicks”; the card is set-up for a 6mm Remington. You will be hand- writing in your information for your specific caliber or loads.
As you can see, I have drawn a rough sketch of the area that I have glassed as well as the indicators or “Target Reference Points” previously mentioned. There is a stream that runs from left to right with a watering hole on the right side of the card around the 700/800 yard mark.
Most of the pertinent data has been recorded which includes the temperature, barometric pressure and wind speed and direction; the descriptions of the TRP’s with their locations and distance.
In the Gravity Range boxes, you can see that I have inputted the holds in MILS for both elevation and windage. This did not take very long to do at all and has prepared me for an immediate shot on any target that walks into the area. The guesswork has been eliminated and replaced by positioning, procedure, preparedness and simple math. Should you possess decent and tuned equipment, you should not miss.