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Tuesday, December 8, 2015

Zero Like A Hero Pt 1: What it Means

If you're going to hit your target, you need to use your sights. For those sights to be effective, you need to make sure they are aligned with the gun. This aligning process is called zeroing, and is critical to accuracy. (Note, everything in this process assumes that you're using a scope or other optic, and that it is properly mounted. How to diagnose and correct mounting issues will be covered next week.)

What Zeroing Is
The first thing to understand when you zero your optic is what your bullet is doing when it leaves the barrel. Contrary to what some folks think, the path a bullet takes is anything but straight. Gravity and air friction obviously make a bullet slow down and fall to earth, but far more interesting is what happens as a bullet leaves the muzzle.
  1. The point where the focal point of the optic and the trajectory of the bullet intersect is called zero.
  2. Because your optic is higher than the bore of your rifle, you have to focus it lower than parallel. This means that your optic's line of sight is essentially pointing down when the barrel is held parallel to the ground.
  3. When you aim at a target far away, your optic's line of sight is parallel to the ground, which means that the bore if your rifle is now pointing up. 
  4. Therefore when you fire your rifle, the bullet seems to climb for a bit,  although in actuality it's following a parabolic arc. It rises for a bit until wind resistance slows it, and then it drops. 
  5. If your rifle is zeroed properly, the bullet will drop and hit where you're aiming. 
So from the shooter's perspective, the bullet exits the barrel, climbs some, then slows down and drops until it hits the ground. In this process, the projectile passes through the crosshairs of your optic twice, once on the way up and once on the way down. This is important, because it gives the opportunity for a very simple zero method: you can zero your optic at a close target (called near zero), where it's easy to see where you're hitting, and that will also give you a zero at a larger distance (called far zero). While I suppose you could just set up and bang away at that range straight away, if you're not already close to zero you might not even hit the paper of your target.

This is where we revisit the up and down path of a bullet, and the simpler zero method I mentioned earlier.

Example: 55 grain .223, one of the most common cartridges in the USA.
A rifle's zero is set at a specific range determined by the shooter and based on a number of factors. The factors that shape this decision include the caliber of the rifle, the intended uses of the gun, and the shooter's skill level. As an example, my .22LR is zeroed at 50 yards. Most of my hunting rifles are zeroed at 200 yards, and both my precision rifle and my wife's are zeroed at 400. Erin zeroed her Mosin at 300 yards, because she was feeling froggy.
  • A good rule of thumb for 5.56/.223 cartridges is that the rifle will do the same thing at 50 yards that it does at 200. This means that if a rifle is dead-on at 50 yards, it should also be dead on (or very close) at 200 yards. 
  • 7.62mm/30 caliber cartridges are a bit different due to bullet weight and powder charge. Erin zeroed her 7.62x54r Mosin 2" high at 25 yards, so it should hit roughly 2" high at 200 yards as well, and dead-on at 300. 
  • 7.62x39, used by SKSes and AK-47, has a dramatic arcing trajectory and does well with a 25/200 yard zero.
  • .22 Long Rifle is obviously not as aerodynamic nor as fast as a centerfire rifle, so it has a much shorter range and zero. Ideally, a .22LR should zero at 60 yards, but 60 yard ranges are awful hard to come by. This is why mine zero at 50 yards, and the difference ends up being negligible. 25 yards remains a good starting point for getting on paper, then making the needed adjustments at 50 yards.
Once you've set your 25 yard zero, move your target out to your desired range and confirm that your rounds are landing where you want them to. You'll very likely have to make some small adjustments, but you should be fairly close to your desired outcome.

A Note on Maximum Point Blank Range
Maximum point blank range is the longest range at which a given cartridge stays with a predetermined circle. In general use, the radius of that circle is the highest point above the muzzle that the bullet reaches. Once it has dropped that same amount, it has reached maximum point blank range. For most .30 rifles, this range happens to be somewhere in the neighborhood of 200 yards.

What does all of that mean? In short, I can hold the crosshairs of my hunting rifles at the desired point of impact at any point between 0 and 200 yards, and hit close enough to that point to have high odds of a clean, lethal shot. It makes quick shots on game animals simple and instinctive. The same logic applies to the .22LR with a 50 yard zero (and actually is the math behind declaring 60 yards to be ideal).

Get zeroed and get practicing.


The Fine Print

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