Another member posted a question in the General Forum seeking advice on zeroing his AR. I responded and decided to post the response here for anyone seeking the same type of info. The response:
"If you never anticipate shooting past 200 yards, the 50 yard zero is the way to go. Check out the below link for data using both 62 and 55 grain 5.56 ammo. You will find the 50 yard zero allows you to hit point of aim/point of impact from 50 to 200 yards. The max rise is about 2 inches at about 140 yards. 2 inches is well within the ability of most shooters and all but match grade ammo.
The 36 yard zero is one used by the USMC to get a 300 yard battle zero using M855 62 Grain ammo. If your choice of ammo goes above or below that grain the point of impact/zero will shift.
http://www.ar15.com/archive/topic.html?b=3&f=118&t=495607
I zero at 100 meters because all targets beyond the 100 mark require the shooter to hold over only and fit the BDC (bullet drop compensated) ACOG Reticle I use on one of my rifles. I have a couple of Ars, one with a 20 inch barrel the other with a 12.5 inch barrel. Both are zeroed at 100 meters. That way no matter which one I choose, past 100 meters I am always holding over to make reliable hits. Below is a pretty good discussion on the advantages of the 100 meter zero. Althought the discussion is specific to "meters", zeroing at "yards" will not change the results enough to have any effect considering the limitations of most shooters/rifles/ammo. I think the 100 meter/yard is the best fit for me, as always YMMV.
http://www.m4carbine.net/showthread.php?107572-Zen-of-the-100-Meter-Zero
Inside of your "zero range", no matter what distance you zero at, you will need to begin to compensate for "Optical Offset". Basically you will aim high a set distance, based on how high your optic is mounted relative to the center axis of the bore of the barrel. If your optic has an absolute co-witness with your irons, you will be aiming about 2.6 high from about 10 yards to press contact distance. If you choose the "lower 1/3 co-witness" your hold over will be 2.8 inches. Many choose to hold over 2.7 inches because it sits in the middle of the "absolute" and "lower 1/3" range. This requirement is a function of the height of the front sight above the bore axis of the barrel. The AR platform with a standard "F" marked (for use with flat top AR upper recievers) front sight base has about a 2.6 inch height difference. The standard front sight base with no "F" mark (for use with fixed carry handle Ar upper uppers) differs in height by -0.04 of an inch, so again based on shooter/rifle/ammo limitations, using 2.6 or 2.8 inches (depending on where you co-witness your optic/iron sights) for both is go to go.
To put it simply, if you wanted to hit a man between the eyes at ten yards or closer, aim high center forehead. From 10 yards back to your "zero range" the holdover distance begins to lessens until it is point of aim/point of impact at the "zero range". Outside of ten yards simply aim where you want the bullet to strike until you are back to your "zero range" distance.
I would add, that no matter what distance you zero at, to be 100% sure you are dead on you must shoot to known distances and make any fine adjustments required to complete your zero. So, for example, if you choose to zero at 50 yards you will to need check your zero at 200 yards to be sure it is on. Zeroing at 25 or 50 yards rarely yeilds a dead on zero at 300 and 200 yards respectively."
I currently have my Vortex Strikefire zeroed at 25 meters. I have been reading a lot about the 36 yard zero. Really don't expect to do a lot of shooting past 150-200 yards, and a little confused on if I should use the 36 yard zero and how effective it would be at closer encounters.
"If you never anticipate shooting past 200 yards, the 50 yard zero is the way to go. Check out the below link for data using both 62 and 55 grain 5.56 ammo. You will find the 50 yard zero allows you to hit point of aim/point of impact from 50 to 200 yards. The max rise is about 2 inches at about 140 yards. 2 inches is well within the ability of most shooters and all but match grade ammo.
The 36 yard zero is one used by the USMC to get a 300 yard battle zero using M855 62 Grain ammo. If your choice of ammo goes above or below that grain the point of impact/zero will shift.
http://www.ar15.com/archive/topic.html?b=3&f=118&t=495607
I zero at 100 meters because all targets beyond the 100 mark require the shooter to hold over only and fit the BDC (bullet drop compensated) ACOG Reticle I use on one of my rifles. I have a couple of Ars, one with a 20 inch barrel the other with a 12.5 inch barrel. Both are zeroed at 100 meters. That way no matter which one I choose, past 100 meters I am always holding over to make reliable hits. Below is a pretty good discussion on the advantages of the 100 meter zero. Althought the discussion is specific to "meters", zeroing at "yards" will not change the results enough to have any effect considering the limitations of most shooters/rifles/ammo. I think the 100 meter/yard is the best fit for me, as always YMMV.
http://www.m4carbine.net/showthread.php?107572-Zen-of-the-100-Meter-Zero
Inside of your "zero range", no matter what distance you zero at, you will need to begin to compensate for "Optical Offset". Basically you will aim high a set distance, based on how high your optic is mounted relative to the center axis of the bore of the barrel. If your optic has an absolute co-witness with your irons, you will be aiming about 2.6 high from about 10 yards to press contact distance. If you choose the "lower 1/3 co-witness" your hold over will be 2.8 inches. Many choose to hold over 2.7 inches because it sits in the middle of the "absolute" and "lower 1/3" range. This requirement is a function of the height of the front sight above the bore axis of the barrel. The AR platform with a standard "F" marked (for use with flat top AR upper recievers) front sight base has about a 2.6 inch height difference. The standard front sight base with no "F" mark (for use with fixed carry handle Ar upper uppers) differs in height by -0.04 of an inch, so again based on shooter/rifle/ammo limitations, using 2.6 or 2.8 inches (depending on where you co-witness your optic/iron sights) for both is go to go.
To put it simply, if you wanted to hit a man between the eyes at ten yards or closer, aim high center forehead. From 10 yards back to your "zero range" the holdover distance begins to lessens until it is point of aim/point of impact at the "zero range". Outside of ten yards simply aim where you want the bullet to strike until you are back to your "zero range" distance.
I would add, that no matter what distance you zero at, to be 100% sure you are dead on you must shoot to known distances and make any fine adjustments required to complete your zero. So, for example, if you choose to zero at 50 yards you will to need check your zero at 200 yards to be sure it is on. Zeroing at 25 or 50 yards rarely yeilds a dead on zero at 300 and 200 yards respectively."
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