Those who carry guns for a living know the world isn’t a nicely manicured flat range and threats will present at differing distances. You won’t have an opportunity to dial in your DOPE to take the shot when you need to. To counter this, anyone not using an optic equipped with a bullet drop compensator uses a battlesight zero or BZO. Before we really get into the BZO let’s look at what the Army and Marine Corps have to say in doctrine.
“The term battlesight zero means the combination of sight settings and trajectory that greatly reduces or eliminates the need for precise range estimation, further eliminating sight adjustment, holdover or hold-under for the most likely engagements. The battlesight zero is the default sight setting for a weapon, ammunition, and aiming device combination.
An appropriate battlesight zero allows the firer to accurately engage targets out to a set distance without an adjusted aiming point. For aiming devices that are not designed to be adjusted in combat, or do not have a bullet drop compensator, such as the M68, the selection of the appropriate battlesight zero distance is critical.” – US Army Training Circular 3-22.9, May 2016.
“…In combat, the Service Rifle’s BZO setting will enable engagement of point targets from 0 to 300 meters/yards in a no-wind condition.” – US Marine Corps Reference Publication 3-01A, October 2012.
Now that we’ve covered what a battlesight zero is, let’s discuss it. The Army used a 200 yard battlesight zero for the M1 rifle, 250 meters with the M16A1 and went to 300 meters with the M16A2. Today, many (most) understand 300 meters to be the absolute standard, and while it is the one most frequently in use, updated doctrine as quoted above leaves room for critical thinking. I spent quite a bit of time looking for the “why” behind the 300 meter zero being the virtual gold standard and asked some of those that would know if anyone I knew would. The best answer I could find is a now-declassified 1952 study on future weapon requirements that repeatedly referenced 300 yards as being about the limit of typical rifle engagements.
I remember being told in Infantry One Station Unit Training (OSUT) over 21 years ago to aim low when engaging targets at 150-200 meters. The explanation was, “the bullet rises off the muzzle”. We had little if any idea what our drill sergeants were talking about so we nodded our heads and blindly aimed low at 150 and 200 meter targets even though we had been taught to aim at the center of visible mass. It was counterintuitive and would’ve been much easier to comprehend with an explanation of line of sight, line of bore, and external ballistics, but we did as told and collectively did marginally well enough to pass. In time I was able to progress from this least common denominator teaching.
Why does this matter? Ballistics. Everything one does in zeroing for unknown distance engagements is a tradeoff. The bullet only crosses the line of sight two places, on the way up and on the way down. Shots at any other distance either require adjusting the sights or using a hold to put the bullet precisely where you want it (or accepting a fudge factor for “acceptable” hits).
Ballistic data puts the trajectory of 5.56mm service ammunition from the M4 or M16 Series weapon at around 6 inches above point of aim at approximately 175 meters on a 300 meter zero. Further, one military training organization says 7-10” (this is likely taking the rifle/ammo combination being a 3 MOA average system into account). A 200 meter zero with the same weapon and ammunition will produce a maximum ordinate of about 2” above point of aim at approximately 125 meters, but the trajectory is about 9” low at 300 meters. A 100 meter zero with the same weapon and ammunition sees maximum ordinate at the point of aim at 100 meters, but the trajectory is about 3” low at 200 and over 12” low at 300.
That means choosing where you want the trajectory and the sight plane to intersect as well as choosing how much deviation between trajectory and sight plane is acceptable and at what distance. This takes knowledge, both of the operating environment and of the weapon and ammunition. Is the 300 meter zero what you want or do you need a 200 or 100?
Some trainers and organizations say stick with 300. Some say 200. Others yet say 100. There are still others out there that say 250. While the standard answer in the US Army continues to be 300, many basic trainees are using 200 as a result of findings by the Army Research Institute. Each has strengths and each has weaknesses compared with the others. All require understanding of ballistics and application of holds to deliver precision fires at distances other than the zero distance.
My take is more of a question or series thereof. What are the ballistics of your weapon and ammunition? Is the operating environment urban or rural, or it is a mixture? Is it heavily vegetated? Is the terrain relatively flat or extremely mountainous? What are the most likely reasonable engagement distances?
What is the best battlesight zero? My answer is it depends. Given my current applications and surroundings I go with a 50/200. Different circumstances could dictate a different zero.