Long Gun Zero - Tactics and Weapons - LawOfficer.com

Long Gun Zero

Where should you set your carbines point of aim/point of impact?

 


 

Jeff Chudwin | From the February 2008 Issue Wednesday, January 30, 2008

Not long ago, I received a call with a request for information related to the AR-15/M-16 weapon system from good friend Mark Schlegel of FTF Tactics. Mark is a long-time SWAT officer from one of the largest sheriff s departments in the country. As a cofounder of his law enforcement training company, Mark and his cadre have provided superior training to agencies and law enforcement associations across the nation, including our own Illinois Tactical Officers Association.

The timing of the call was on the mark as we had just completed instructing the first day of our AR-15/M-16 armorers class. Top gunsmiths, writers and shooters Ned Christiansen and Pat Sweeney had come to Illinois to assist me with this in-service class. Both have decades of experience with the AR-15/M-16 system. The class materials included a study of patrol-rifle sight systems and settings.

Schlegel explained he was working with a SWAT team in a southern state. The team officers were concerned with entry work that s often dynamic and close, and wanted to know how best to regulate the sights on their M-4/CAR-15 type weapons. Meaning, at what distance should they set the sights so that the bullet strikes the visual point of aim? This is known as zeroing the weapon point of aim equals point of impact.

Why, they asked, shouldn t they set their weapons for zero at the distance across a residential living room? This is the distance they work in when doing warrant service and other entry-type work. The team had read various materials that discussed zeroing, including the military manuals regarding the M-16 series rifles. Knowing that our training group has spent considerable time studying this issue, Schlegel asked if we could shed some light on the ballistic and mechanical facets.

We happily obliged. Here s the gist of our response.

The Nuts & Bolts

Let s first review the mechanics. Note: The AR-15/M-16 system is the dominant type in use, so I will use it for reference. The principles remain the same for other weapon types.

The M-16 system was designed with the tip of the front sight 2.5 inches above the bore s centerline. If you were to press the muzzle of the rifle against the target, the bullet would therefore impact 2.5 inches below the line of sight. By raising or lowering the front sight, we can adjust the bullet impact up or down.

Unlike the original M-16A1, there s an elevation knob on the M-16A2 rear sight. We choose not to adjust elevation from the rear sight and lock the rear sight down to the base and leave it there. Raising the front sight lowers the point of impact, and lowering the front sight does the opposite.

At some point in space, the bore line will converge with the line of sight. How we make this happen and why is the issue. Because the sights sit so high above the bore, there will be a limit on how close we can zero the sights. And even if we can mechanically get an across-the-room zero, is that workable for our needs?

The short answer: No. Allow me to illustrate.

The 7 Yard Zero chart shows the difference in line of sight/line of bore from the muzzle to 250 yards. Considering the SWAT officer s request for room-length zero, here s the problem. A 7-yard zero, even if mechanically possible, would cause the bullet to impact high above the point of aim as follows: 2.5 inches at 14 yards, 7 inches at 25 yards, 14.5 inches at 50 yards, 24 inches at 75 yards, 30.5 inches at 100 yards, 45 inches at 150 yards and 58 inches at 200 yards.

So, an officer deploying a rifle set to a 7-yard zero would have to know and remember under high stress to aim low or miss a shot that could travel two to four feet above the point of aim. A 75-yard hallway shot in a school or office building with this rifle would impact two feet above the point of aim. Obviously, that s unacceptable.

If you review officer-involved shootings that feature a rifle deployment, you ll find distances are typically short-range, such as the length of a room or across a car or roadway. That s not to say all are close quarters, however. Engagements 25 100 yards away and more have occurred nationwide.

We carry our patrol rifles in part to respond to active-shooter incidents, and school hallways and shopping-mall plazas can easily exceed 75 yards in length. Therefore, we must have a sight system that gives a patrol or SWAT officer the ability to aim and fire directly on target from 0 200 yards without having to remember multiple points of aim due to target distance. This gives an officer a high degree of confidence of achieving hits on target.

The military uses a zero method that allows engagement well beyond 300 meters, but that s not the expected law enforcement work environment. Officers must be able to first identify a deadly force offender before firing. With the naked eye, it s very unlikely you can ID a threat much beyond 100 yards.

For this reason, we ve chosen to set our zero at 100 yards.

The 100-Yard Zero

After testing many of the zero systems and working with the ballistics of the .223-caliber/5.56mm round, we train our officers to adjust their sights so their weapon impacts 1.5 inches below point of aim at 25 yards, achieving a 100-yard zero. This is based on the AR-15/M-16 type rifle or carbine firing a 55-grain bullet at a conservative muzzle velocity of 3,000 feet per second.

From 50 75 yards, the bullet impacts mere tenths of an inch below the line of sight as it rises to center at 100 yards. As the bullet travels further, loss of velocity and the effect of gravity pull it down, and it again falls below the line of sight. At 200 yards, it impacts approximately 2.5 inches low. So, from contact distance with the target out to 200 yards, the bullet never rises above the line of sight and doesn t fall more than 2.5 inches below the line of sight.

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UPdated25yrdSightInTarget2.pdf17.45 KB
_yard zero chart.pdf39.95 KB



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Jeff ChudwinJeff Chudwin is the 2009 Law Officer Trainer of the Year, serves as chief of police for the Village of Olympia Fields, Ill.

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