Precision Shooting 1-Part 2: Optics, Data and Logic
Rifle Scopes – The Good, the Bad and the Ugly
Over the Christmas holidays a good friend and gun writer “Charles” came out to Los Angeles, California for his holiday visit with his daughter; and with him came the new Armalite bolt action rifle, chambered in .338 Lapua. I picked him up around 9:00 a.m. and we proceeded to the local outdoor firing range. As I had made some adjustments to my rifle, I was eager to check the zero of my $800.00 scope. We ran the ceremonial patch down our rifles bores, and then setup at our individual benches; Charles was shooting the steel targets starting at 200 hundred yards and ending at 700 yards. It was hard not to pay attention to Charlie as every time he pulled the trigger, you would hear a humongous blast from his muzzle break followed by his 300 grain bullets slamming into the steel targets with a ton of power.
As for me, I set up on paper that was 100 yards away. I attempted to zero, but the ¼” clicks of my elevation and windage turrets appeared to be way off. For one, I was about ¾” high above the bulls-eye, so I turned down the elevation 3 clicks and then rapped on top of it to make sure that the reticle was seated, and then fired again. The next round was 1-3/4” lower than the first point of impact. To make a long story short, the interior components of the scope had somehow gotten out of whack, way out of whack. What also occurred is that the parallax adjustment knob became very difficult to turn. My hold for 700 yards (for the current weather conditions) was 4.4 mil radians. Upon engaging the target, my hold was now at 3.9 mil radians which meant that the calibration of the reticle was now off as well; and that equates to twelve and one-half inches (12.6”) at seven hundred yards. This does happen more frequently then you would think with a non professional grade scope. The difference is that as a professional, a Law Enforcement officer or Military Sniper, you notice these things because your life and or the lives of others depend on you and the equipment that you are using. As a hunter that never shoots beyond three hundred yards, you might just except this as a fact of life and keep shooting with it.
Next to the rifle, the telescopic site is the most important accessory and is one reason why some hunters may not have the confidence to shoot beyond three hundred yards. I may have made mention that military snipers and designated marksmen are taught not to engage a target any closer than 300 yards. So what is the difference here? Are military snipers any better at their shooting skills than hunters? No, not necessarily. But the military shooters do, for the most part, have good equipment that stands up to hard recoil and tough field conditions, and they practice a lot. They are in tune with their equipment and replace or repair it immediately if and when it begins to go awry. In my case, I was utilizing a scope that has a reputation in the industry for not having the best quality control. As the performance was on a steady decline, I was wondering when it was going to finally shake loose. I shoot at least once a week, so I know what is going on with my equipment. But some people go out once a year, check their zeroes before leaving for a hunt, hopefully obtain their quarry and then put their rifle back in the gun cabinet or safe for another year. If the gun shoots a 3 inch group at 100 yards, they feel that that is all they need and in my opinion, they could not be more wrong.
Scopes are not very simple pieces of equipment. After all, they are precision “telescopes” and in the case of Night Force, Schmidt & Bender, Leupold, and Hensholdt, precision assembled. If they are going to take the shock of recoil or being dropped and banged up, and still be able to function flawlessly, they have to be manufactured very well and consist of exceptional components. After all, there are some people that stake their lives and the lives of others on them.
Without going into too much detail, rifle scopes are manufactured in some cases with up to eleven lenses. Inside is in an erector tube cassette that houses the erector lenses, and in front of that, the side mounted parallax adjustment. The erector lenses are utilized to correct the image (upside down). This “latter day” method of building scopes is not without inherent problems that can and usually will present themselves when utilizing a scope of poor quality; let me explain. The interior of the scope tube houses the lenses, parallax and erector cassette/tube. The interior dimensions are critical to the successful operation of the scope and what becomes of utmost importance in addition to the springs and glass utilized, is that known as its ocular center.
When you are looking through your scope at the cross-hair, you are looking through its ocular center. When you zero your rifle for medium ranges the interior dimensions and clearances must not be compromised. However, when you mount a sloped base on top of your receiver, and zero your rifle for lets say six hundred yards, you have pulled the erector tube up and closer to the interior wall of the scope tube itself. If left in this “tilted” position, I believe that it is appropriate to say, that the springs (if not high quality springs or components) holding the erector tube will one day fatigue, compromising the interior dimension between the exterior of the erector tube and the interior of the scope tube, causing erratic adjustment behavior of the turrets or the reticle to come out of calibration altogether; and this is what I believe occurred that day at the range when my scope failed. For this reason, I believe that the most dependable scopes manufactured today are those with a centrally located cross hair reticle, and designed with Quality in mind.
The exterior dimension of the tube also plays a critical role when shooting at longer ranges. A 30mm diameter tube allows more “minute of angle” adjustment than a one inch tube; and a 34 mm tube allows for even greater adjustment.
There are basically two ways to properly use and or adjust your scope when targeting. One way is to turn or adjust your turrets to the predetermined amount of minute of angle and the other way is to hold on your mil-dots.
Minute of angle equals 1.047 inches at 100 yards. Let us for a minute assume that we are utilizing a ballistic targeting software package such as Exball. We input the current weather data along with our zero, the bullets velocity and ballistic coefficient and then input our distance to target. Let’s say that the distance to target is 500 yards and again, we are shooting a .300 Winchester Magnum, 210 grain Berger Bullet moving at 2814 feet per second. The software tells us that in order to hit our target that the correct site height above the bore needs to be 8.7 minutes of angle. (8.7 X 1.047 = 9.1089) X 5 = 45.5 inches of drop). So we would adjust our elevation turret (come ups) to 8.7 minute of angle, if we are shooting flat. If you choose to adjust your turrets and utilize “Minute of Angle” there is a proper way to do accomplish this and it is very important.
When adjusting your windage or elevation turrets, you should always index the turret from the same direction because of screw run-out.
Screw run-out is caused by the “gap” in the threads and the pitch of the threads on the screw. The more precision the screw, the less “gap”, however, there will always be some. This not only occurs in screws, but also in precision CNC machines, precision drill presses etc…, and (in my opinion) must be addressed when adjusting the turrets of your riflescope.
When run-out occurs, the “per click” value of the adjustment turret changes. For example, let’s say that at one hundred yards, the click value of your scope is set at .25”. However, when adjusting the turret to its far end of adjustment, this value could change to perhaps .23”. Correcting for this problem actually addresses and potentially solves two issues; 1) The gap in and pitch of the threads of the screws which cause run-out and; 2) the possibility of sticking reticles.
Let’s say that your shot requires you to adjust your elevation to nine minutes of angle. To prevent screw run-out and a potentially sticking reticle, you would rotate the elevation turret until you reach an indicated eleven minutes of angle, then rotate the elevation turret back down to nine minutes of angle. This causes the backlash to be taken out of the screw threads, preloads the springs holding the reticle or reticle cassette, (or erector tube) and pulls the reticle (or erector tube) into the correct position. Indexing from the same direction is very important. This is why a great many instructors and experienced shooters who have had this experience, will always rap on top of the turret to hopefully insure that the reticle has seated properly.
In addition, there lies another anomaly that you should be aware of. If your scope has a total of forty minutes of angle (moa) of adjustment that would theoretically mean that from center, you have twenty moa up and twenty moa down; that is all you have. If you have ranged your target and find that you need twenty-one moa to hit your target it isn’t gong to happen. Even though your turret may continue to spin and click, there is no more adjustment left and nothing mechanically is happening.
Mil Radians are a world wide military standard method of measurement and is defined as a unit of angular distance equal to one thousandth of a radian. The “RAD” or “Radian” is a unit of plane angle adopted under the Systeme International d'Unites; which is equal to the angle at the center of a circle subtended by an arc equal in length to the radius. That’s a lot of French to chew off I know, however we are only concerned about how to use it, which is simple. The “mil-dot” reticle was developed in the mid 1970’s for the main purpose of estimating range (distance to target). In addition, the mil-dot reticle gave a whole new meaning to the phrase “hold over”.
As it is a standard method of measurement, there are 6,283 Mils in one 360 degree circle; each mil radian mark/dot on the reticle equals 3.6 inches at 100 yards or 10 centimeters at 100 meters. To use the mil dot reticle to estimate your distance to target, the two equations are fairly simple.
Equation #1: Take the size of your target in yards, multiply it to 1000 and then divide by the number of mil-dots your target fits in. As an example, let’s say that you are hunting deer, and the distance from the bottom of the belly to the top of the back equals 18 inches; 18 inches is ½ of a yard. So the equation would be: .5 X 1000 / mil radians = Distance to target.
Equation #2: Take the size of your target in inches, multiply it to 27.77 and then divide by the number of mil-dots your target fits in. As an example, let’s say that you are hunting deer, and the distance from the bottom of the belly to the top of the back equals 18 inches; (18 inches X 27.77) / mil radians = Distance to target.
Holding over is another means of using your reticle to aim at your target. Several paragraphs ago, we discussed the minute of angle adjustment for a 500 yard target. When aiming, we can choose to adjust our turrets for “minute of angle” or “hold” on a specified mil-dot mark or minute of angle mark depending on the type of reticle we are using. Utilizing the same data for our 500 yard target, our mil-dot hold over is calculated to be 2.5 mils. This means that we would hold on line (mil-dot mark) 2.5 instead of adjusting our turrets. While working for a scope manufacturer / distributor once upon a time, I coined the phrase, “when your fingers are clicking, the clock is ticking”. It is considerably faster to use the mil-dot method hold over then to spin your turrets, but not necessarily as accurate. In a military environment with time not on your side, mil-dot holds are the quickest.
There are some reticle designs available today that utilize the mil-dot method with the mil-dots broken down into ½, ¼, or 2/10th of a mil. These are pretty accurate and very fast on target. As an example, let’s imagine that we are hunting Elk together and we are glassing a canyon. Using our binoculars, we spot a Bull and several Cow’s pretty far off in the distance, in fact as I look down the canyon at them, they look really far away. You say to me, “it looks too far and that perhaps we should figure out a stalking approach”. This is because we are looking downhill at them. When you look downhill, the target always looks further than it really is. The opposite holds true when you are looking uphill; the target most always appears to be closer than it really is. In addition, the terrain can fool you because there are no real world objects of size to use as a reference. Sometimes four hundred yards can look like a mile and then again, it can look relatively close.
Moving along, we slowly squat to a kneeling position while I slowly take out my Leica Rangefinder and range the distance; they are 679 yards and the wind is calm; I will take this shot. But before I do, I look at my “Angle Cosine Indicator” and obtain the cosine number I need to multiply to my moa hold, to obtain the corrected for gravity distance; .8 is indicated. At this point I do the math, (.8 X 14.0 moa (679 yards) = 11.2 moa; a correction of 2.8 moa. If I hadn’t corrected, I would have hit approximately 19 inches high. If I use my Pocket PC with the Exbal software installed in it, the immediate answer of 10.5 moa is delivered. This is a total difference of 3.5 moa for a total miss of 23.7 inches high. Obviously, if I hadn’t corrected for gravity, my bullet would have hit high and I would have boldly missed.
With my hold in mils or minute of angle established, I get as comfortable as possible, obtain as secure a platform as possible for my rifle; aim, breath and squeeze the trigger.
Without the aid of the ballistic targeting software, this would be the math that I would use to obtain my hold and it really boils down to a few steps. The method works for both mil-dots and minute of angle holds.
1) Obtain the distance to target. Let’s say that it is 500 yards.
2) While aiming at your target, obtain the cosine number by looking at the ACI. Let’s say, that the indicated cosine number is .70.
3) Look at your data card and obtain the moa or mil-rad hold for 500 yards, (which is 10.2 moa), and multiply that by .70c. So the equation would be as follows: 10.2 (moa) X .70c = 7.14 moa.
4) Now make your adjustments.
“Math is math and science is science.” You can only argue facts if your mind is filled with fiction.