Having spent time in the military and working at a public range I learned that most people don’t understand zeroing sights or optics.
I always found it humorous when some of the regulars at the range would have a new rifle or new optic and invite me to come over and try their new gun. I’d fire a shot and tell them something like, “Cool rifle, but it is impacting 4 inches left for me.” The response I would get would be a coy line much like, “Well why don’t you go ahead and dial it then.”
I like to imagine that I keep all my firearms combat ready, but realistically I would never choose to use some of them in a fight. I wouldn’t grab the 10/22 for obvious reasons. While I would trust a Garand in a fight, it would be far from my first choice. One of the most critical things I think of as part of being “combat ready” would be the simple ability to hit what you are aiming at.
I like to think of the quality of a zero on a firearm as one of several states. I don’t think I’ve seen other people talk much about this, so I want to lay out what I think it.
- Mechanical Zero
- Battle Sight Zero (BZO) or Reduced Range Zero
- Fine Zero
- True Zero (or proofed zero)
Unzeroed: The least desirable state for a firearm sights to be in. Hopefully an unzeroed firearm will impact close to where you aim, but there is no way to know with out test firing or checking the bore axis to the sights against a common index.
Mechanical Zero: The sight is centered either mechanically or optically. On something like a micrometer adjustable sight, mech zero may be obtained by counting the total number of clicks and adjusting it half way. On scope you could count clicks or use a mirror to get the crosshair centered in the tube. Centering a scope via scope adjustment may not be the same as optically centering.
Generally, one a well built firearm, mechanical zero will be close to right on. On cheaply built guns, not likely. If you have something like a rifle with a 30MOA canted base for long range shooting, the mechanical zero on the scope will deviate from a proper zero because of that.
It used that a brand new, out of the bolt, Colt AR15 or M16 generally didn’t need adjustments from mechanical zero when sighting in. But as of the last few years this no longer seems to be the case.
Battle Sight Zero (BZO) or Reduced Range Zero: There are all manner of reduced range zeroing techniques. Rarely you will see 10m zeroing targets. Often reduced range zeroing in militaries is done at 25m. For the longest time the USMC liked to use 36 yards for a reduced range zero on the M16A2/M16A4. The idea of a reduced range zero is to easily reproduce a longer range fighting zero at reduced ranged. It is easier and faster to zero at 25 meters than 300. Negligible effect from wind, easier to change and inspect targets, etc. The downside is that ANY minor error at this reduced range will be magnified at farther ranges. Say if a soldier was impacting 1 inch left at 25 meters, they might completely miss a hostile enemy at 300 meters. That could cost lives.
I consider a BZO an acceptable zero. I’ve found that with a 14.5 inch AR15 firing M855, if I impact 0.3 inches low at 25 yards, I will be right on at 300 yards. This lets me quickly and easily sight in any similar carbine at the very common distance of 25 yards. I used to have a scoped rifle where my 100 yard zero was 2.6 mils different from my 25 yard zero. I could dial up 2.6 mils and be right on at 25 yards. This allowed me to double check that zero with that gun at reduced range.
A BZO often won’t give you a perfect zero for the farther distance, but they should be close enough. The now common 50/200 zero is a good example. Zeroing at 50y or 50m isn’t going to give you a perfect dead on zero at 200, but it tends be close enough for practical work.
I would not hesitate to go into combat with a firearm that has a BZO. I would prefer a finer zero, but a BZO is functional.
Fine Zero: Simply put, a fine zero is zeroed at the range the firearm is intended to be sighted in at, and is adjusted as closely to being perfect as possible. A magnified scoped rifle might be fine zeroed at 100 yards. Something like a M16A2 or M4 Carbine with CCO would be fine zeroed at 300 meters. Often people going into combat never get the chance to fine zero and must just rely on a BZO.
Sometimes you are limited by the precision of the adjustments. As an extreme example, the leaf sight on my M203. Each click of the windage knob adjust the impact by 1.5 METERS at 200 meters. So if I fire a shot and impact 1/2 METER left of my point of aim, I can’t adjust closer than that. But a fine zero will be as accurate as precise as the sights allow.
A fine zero is preferred over a BZO as it will have removed any error from the BZO and have been tested out to the preferred sighting distance.
True Zero or Proofed Zero: You don’t tend to hear about this outside of precision shooters, longer range hunters, or snipers. People who have to shoot at multiple distances, or an unexpected longer range distance may take the extra step to true or proof their zero.
This is less about the zero, and more about the knowledge and preparation the shooter has made for long distance shooting. Truing or Proofing is finding out where exactly you will hit or the adjustment you need for the various ranges you might be shooting.
Simple example, I used to shoot 565 yards with a 4x ACOG. 565 yards is about 516 meters, so I should have been able to use the 500m mark in the ACOG. Instead, with my firearm and ammo I needed to use the 600m mark to impact where I wanted to hit. Had I only relied on the stock marking I would have always missed.
While I was in the Corps, when we shot the rifle range, we would note our true zero for each distance. Windage adjustment might change due to how we held our rifles, elevation might be slightly different as well. So one persons 500 yard zero might have been setting the rifle rear sight to 5, another might need to set their A2 rear sight to 5 plus 1 click.
You might have a great gun with an a great cartridge and your ballistic calculator spits out a hold over for some distance, but when you actually shoot that distance you may find you need a different adjustment or hold over. Accounting for that is truing or proofing your zero. With out proofing, that drop chart or BDC is just a suggestion, not a fact.
A very few ballistic calculators give the ability to put in your proofing results to calculate a corrected drop chart to ensure you will hit when you need to hit.
Not that simple, isn’t it?