This posting is for the person who has experience shooting his rifle, but may not quite understand the science behind getting the rifle properly zeroed or "sighted in". While there are numerous variables behind it, the process is fairly simple once you understand the basic theory. Now that we have the interwebz at our fingers, it's even simpler than ever.
The use of a ballistic calculator is one of the greatest tools to use for the purposes of determining what range you should zero your rifle at. For demonstration purposes, I will use the HK91 as an example to show the variables you will need to plug in. I know most hunters are "traditionalists" and use bolt-action rifles with the standard-curve stock, but the fundamentals here are the same and I just like using my HK...so enough about that. Here's the picture.
Now for the variables...1) First and foremost, you're going to need to know what kind of ammunition you are using. Ammunition will have a specific bullet weight and type, plus a specific nominal muzzle velocity. This is typically marked on the ammunition box, but can easily be found online when looking for different types of ammunition to compare.
2) Sight height is another necessary piece of information. See those two red lines in the picture? The top line represents the centerline of the scope, while the bottom line represents the centerline of the barrel's bore. Scope and bore, under 99.44% of circumstances, will be parallel to each other. The exception to this is when using a long-range setup with a very heavy projectile, such as a .50BMG...needless to say, you're not likely to encounter this when hunting white tail or elk.
3) Maximum range is also important. First, discover the maximum distance of any shot you are likely to take on your individual hunting grounds. For example, in the Piney Woods of East Texas, tree coverage makes it next to impossible for most people to take a shot farther than 80-90yds. In the deserts of West Texas, an open line of sight for up to a mile or more is not necessarily uncommon. You need to determine not only the maximum range allowable by your environment, but also the maximum range allowable by your equipment and individual marksmanship skills. Determine the maximum length of shot you're likely to take, and then add an extra hundred yards just to get an idea of what the bullet is going to do.
4) Other variables that will be plugged in are atmospheric conditions. Altitude, barometric pressure, temperature, etc. affect the path of the bullet. Thankfully, at the relatively short ranges most hunters are shooting, these variables aren't nearly as important as the others. Guestimations of what the weather will be like during hunting season should be good enough here.
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Now comes the mathematical portion of the lesson. The rings and mount used to affix the scope to my HK rifle place the sight height at exactly 3" over the bore of my barrel. I use the standard 7.62x51 NATO 147gr FMJ ammunition the rifle was designed for (it's cheaper than boutique hunting rounds, and did I mention it's CHEAP!?). I plugged in my weather and altitude variables.
For a given ammunition type, rifle, and atmospheric condition, the trajectory arc of the bullet will remain constant. The goal is to determine A) what the trajectory arc is, and B) where your scope's line of sight will intersect with the trajectory. In the diagram above, the bullet's trajectory is the black arc, and the scope's line of sight is the green line.
Theoretically, it is possible to have the line of sight intersecting the trajectory arc at any distance between the muzzle and the maximum range of the ammunition. The tricky part is to determine where it is most usable. In the diagram above, notice how the line of sight intersects the trajectory arc twice...the bullet passes line of sight in one direction, arcs downward, and then passes line of sight in the opposite direction.
The goal is to determine the flattest arc possible within your maximum usable range, by zeroing your rifle at a distance that will provide the least variation in point-of-aim/point-of-impact. The reasoning behind this would be the ability to know that no matter what distance within your given hunting area the game may be, you can aim for the vitals and still hit your target within a few inches without having to do any type of guesstimated field corrections in your head. It's a lot easier to remember to breathe and shoot properly, when you're not having to simultaneously do math on the fly. Do your math in the off-season, and you don't have to do it in the deer stand!
Using the rifle and ammuntion I described above, I have determined that a 60yd zero will result in a bullet trajectory that is within 1.6" of my point of aim, from 45 to 250yds. In other words, so long as the wind doesn't get stupid and my aim is good, I can hit a baseball at that distance if I am for the center of it.
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Changing any of the three main variables I've listed will have dramatic consequences on point of impact (POI). For instance, changing the zero distance from 60 to 70 yards will drop the POI almost an inch at 250 yards. Using the same zero distance of 60yds, ammo, and a rifle such as a bolt-action (or semi-auto with the barrel over the gas tube and/or cocking tube, such as the Browning BAR, M1A, FN/FAL, etc) that drops the sight height to 2" will drop the POI down several more inches.
This is why it is absolutely important to know your rifle, your ammunition, and how far you anticipate being able to take a shot.
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After you've figured out the variables on your particular setup, and determined what your optimum zero range is, you need to field-test. Unless you happen to own (or know someone who does own) several acres of land outside of city limits where it's legal to shoot, you'll likely be going to an actual shooting range to shoot during the off-season. One of the easiest and least-expensive methods I've found is using a laser bore-sight prior to going to the range.
They are available in two different types, one mounts inside the muzzle and one mounts inside the chamber. While the chamber bore sight has the advantage of being more accurate, it is limited by the fact that it is only useful for the specific round and the child-rounds derived from it. For instance, a .308 chamber sight will work for .308/7.62 NATO, as well as the .270, .243, and all other "necked down" rounds using the .308 as its base. A muzzle-mount bore sight, on the other hand, is typically universal and comes with caliber-specific arbors to fit a variety of different calibers. Universal muzzle-mounts can be purchased at Walmart and other big-box retailers, while chamber-mounts can be purchased at gun stores, Academy, and other sporting-goods stores. I personally recommend the NC*Star brand, because they are inexpensive and they work. Here is an example of the .308 chamber sight.
Start by using a known range. For example, let's use a 20yd range set up in the back yard. Using your ballistics table for your particular rifle setup and zero distance, determine where your POI should be at 20yds. Using my HK91/Winchester White Box ammo/3" sight height/60yd zero, my POI will be at 1.8" over my point of aim.
After marking the bullseye of the target (I recommend using a standard paper plate and a Sharpie marker), I need to also measure a point 1.8" above the center of the bullseye. My laser dot should be dead center of the bullseye, while the crosshairs of my scope should be dead center in the dot I've made above the bullseye.
Now that you've adjusted your scope with the boresight, you need to do some *actual* field testing, just to ensure that you're where you need to be. The back-yard boresighting is just enough to get you "close enough". Going back to my ballistics table, I see that my POI at 100yds is 1.2" below my point of aim. To test your sights, you can either aim for bull and measure the distance of your impacts, or aim above bull and see if they hit bull.
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When accuracy testing on the range, I recommend repetition. People speak of firing 3-shot groups, but experience shows me that five-shot groups are where it's at. Shoot at 100yds, giving you the benefits of both extended-range over your backyard bore sighting and the ease of knowing that 100yds isn't an extremely long shot. Between shots, mark off two minutes, allowing for a small amount of "cooling off" for both you and your barrel. Make sure you use a bench rest, bipod, sandbags, or whatever else you feel comfortable with, to ensure that your shots are not influenced by anything other than the rifle and ammo.
After each five-round group, take the opportunity to see where your POI is. Look for the center of your group*. If your grouping isn't where your ballistic table says it should be, now would be the time to correct your scope and try again. It's generally good to wait 15-20min between strings, to allow proper cooling of your barrel.
Another thing to remember is that if you are going to be hunting during the winter and range-testing during the summer months, run two calculations...one for each temperature. Run your cold-weather table to determine where your zero point should be, and run your warm-weather table to give you proper POI measurements. Remember high-school math? For a given equation, the constants remain the same, the solution is determined by the variables.
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Online ballistic calculators are available from a variety of sources. One of the best available is the Winchester Online Ballistic Calculator, as it provides a graphic output of the bullet's trajectory in addition to the ballistics tables. Unfortunately, it has two drawbacks. First, it will only calculate Winchester ammunition. Second, it runs on Microsoft Silverlight, and users of Ubuntu and other Linux-based operating systems are not supported.
For those of us who do not run a Windows machine, or who want the ability to plug more intricate variables into the calculator, there is the JBM Ballistics trajectory calculator.
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Good luck, and good hunting.
*If your five-shot groups are patterning more than 2" at 100yds, there is a serious problem. Either you are using a defective rifle, you are using defective ammunition, or you are a defective shooter. If it ain't the ammo or the gun, disregard this and start putting in more range time...
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