Zeroing
I've been seeing a lot of folks arguing back and forth about the best way to get a 300 meter "zero" with their AR15 (or other firearms) at 25 meters.
First off, I define a zero as being one of the possibly two points where the bullet's trajectory and the sight line coincide. If I want to zero at 300 meters then I get my butt out to 300 meters and sight-in. If my range is limited, say to 100 yards, but I still want a 300 meter zero then I'll adjust my 100 yard point of impact so it's above my aiming point. But I call that "sighting-in" or a "sight-in" because it's an estimate- possibly an accurate estimate- rather than a true zero. If I've previously zeroed at 300 meters and then fired a group at 100 yards to get my exact hold-under, then it should be just about spot-on.
The problems really occur when people try to sight-in at extremely close ranges such as 25 meters. Trying to adjust your sights to zero at the range where the bullet trajectory first crosses the sight line (later it will fall past the sight line at a longer range) will only work if your trajectory exactly matches the ideal as outlined in whatever manual you may be reading or calculated by whatever program you may be using. Variations in atmospheric conditions, muzzle velocity, ballistic coefficient (which can change with the same bullet design out of different rifles), and even how the shooter allows the rifle to recoil will change the ballistic path. Tiny errors at 25 meters will magnify greatly at longer ranges.
Close range sight-ins can also mask shooter marksmanship errors. If you pull your shots a tiny bit to the right, at 25 meters you will be able to adjust your sights to compensate for this. At 300 meters the error will grow beyond your weapons sight adjustment range and you'll wonder why you're on at 25 but off the paper at 300. Notice that the reverse is not true; small errors at 300 meters will be vanishingly tiny at 25.
So, I'll give you the method I use to sight-in a weapon and then give you some close-range field expedient sight-in estimates for the AR15 series.
The first thing I do is inspect the sights on the weapon. Unless I'm working on a previously zeroed weapon I will return the sights- optical or mechanical- to their mechanical centers and check their adjustment functioning. I’ll also check the mounts to make sure everything is tightened properly.
Next, I will check for any gross visible misalignment between the sights and the bore and correct that with machining, shims, whatever.
If I have an optical collimator, I'll place that on the weapon and check to see if the sights are parallel to the bore. If there's a major adjustment necessary I'll fix the problem. If they're pretty close, I'll adjust the sights to bring them parallel to the bore. I prefer the collimators that have a grid instead of just an "X" so I can check to be sure the sights adjust in the direction and increments they're supposed to. For example, I ran into a number of scopes made overseas that were marked for quarter inch adjustments but were built to metric standards so really gave three clicks to the inch at 100 yards; not a problem but could give a bit of a surprise on the range. I also firmly tap scopes while they're on the collimator to check for a loose reticle.
Next, it's off to the range. I always work off of a solid bench with a firm front and rear bag. If I'm just checking an already established zero, I'd prefer to work from the prone rather than off something unstable like the hood of a car or a wobbly bench. (Note on car hoods: I've seen several windshields cracked or shattered by the muzzle blast of centerfire rifles, not to mention damage to the paint.) Large bore rifles- anything above .300 magnum for me- or powerful spring airguns should be fired with a fairly firm grip on the forearm and the back of the support hand bedded in the sandbag or the recoil off the bag will make your point of impact radically different from what you'll get in the field from standard positions. A tight sling may also change your point of impact. AR15s are notoriously bad about changing point of impact with minor changes in pressure on the forearm so be aware of that. Also, if your rifle has a sling swivel stud on the forearm, make sure the stud is far enough forward of the sandbag that it won't run into the bag under recoil and make your shot go high.
Before I start shooting I will double check my bore-sight by physically bore-sighting on the most distant object I can find; some weapon mechanisms make this impractical. I sight through the bore at a distant point object- in my back yard I use the radio towers on a nearby mountain. Make sure that the aiming point, the muzzle, and the breach end of the bore are all perfectly concentric. Use the sandbags to lock the rifle in this position. Without disturbing the rifle move up to the sights and check to see if they're on target. If you used an optical collimator don't be surprised when the sights are off. I always had much better results by bore-sighting on the range which is why I try to do it if possible. Check back through the bore to make sure the weapon hasn't moved and make any necessary sight adjustments. Repeat the boresight procedure until the sights and bore are parallel.
If you're really brave you can start your sight-in at 100 yards. I only had a couple of rifles that were off the paper after a field bore-sight but one of them was way off. Just remember that when bore and sights are parallel the point of impact is going to be quite low at 100 yards (or whatever range you choose). Unless ammunition is at a premium I prefer to start at 25 yards.
If I'm relatively confident in the weapon and sights I will shoot one shot and make adjustments. With an unknown weapon or if I'm just out of practice I shoot groups of at least three to five shots before making any adjustments. I zero the weapon at 25 yards. With weapons with a very high sight line such as see-through scope mounts or AR15s I will adjust my point of impact about an inch low at 25 yards so it won't be too high at 100. When I get a good 25 yard zero I move back to the next range. I personally use a 50 yard zero with pistols, slug guns, and most of my .22 rimfires. For my other rifles I will move back to 100 yards.
I will zero my rifles at 100 yards and then make any adjustments I feel necessary for longer ranges. An inch and a half to two inches high at 100 will usually give you a pretty good 200 yard zero. If you're limited to 100 yards this will have to suffice, otherwise I prefer to move out to whatever long range I want to zero at and get a true zero.
Here are the close range zeros for the AR15. Remember that these are estimates- there's no substitute for zeroing your own rifle at the actual range. The military's 25 meter sight-in used to be considered field-expedients where a longer range was unavailable.
The Army method for the old style A1 sight was to flip up the long range sight and zero at exactly 25 meters (not yards). When the short range sight is flipped back it will supposedly be zeroed for 250 meters.
The Marine Corps has gone through several methods for obtaining a battle sight zero with the A2 sight at close range. Their first method was to set the rear sight at 300 meters plus one click and then zero at 25 meters using the front sight to make elevation changes. When the sight is clicked back to 300 it's supposed to be spot on. Somebody at AR15.com ran the numbers on his computer and found that this was incorrect. This close range sight-in actually gets you a 400 meter zero. The Marines discovered the same thing and later changed to a 30 meter (36 yards) zero with the 300 yard setting.
According to the current manuals a 25 meter near zero equals a 375 meter far zero, 42 meters equals 250 meters, and 36 meters equals 300 meters.
Here are the number I ran up on my computer. I found several different ballistic coefficients for both the M855 (62 grain) and the M193 (55 grain) so I used an average. My numbers are pretty close to the military's. I also adjusted for my higher elevation- 7500 feet. I ran sets of numbers for both cartridges and in yards and meters. I ran numbers for all the standard long range zeros as well as a point-blank zero that doesn't allow for the bullet to rise more than 2.5 inches above the line of sight. The computer spits out numbers to two decimal places which is ridiculous for sights that only make MOA or half minute adjustments but it won't hurt anything as long as I don't try to make adjustments to thousandths of an inch.
These numbers are all how many inches above your point of aim the bullet should impact at 100 yards to get the long range zero listed.
For the M855 or SS109 round-
200 yards = 1.15 inches
200 meters = 1.56
239 yds (+2.5 inches to -2.5 inches point blank) = 1.98
250 yds = 2.24
250 m = 2.73
300 yds = 3.53
300 m = 4.38
375 yds (for the A1 long range aperture) = 5.77
375 m = 6.81
For the M193 round-
200 yds = 0.89
200 m = 1.25
250 yds (+2.5 inches to -2.5 inches point blank) = 1.85
250 m = 2.29
300 yds = 3.00
300 m = 3.76
375 yds = 5.03
375 m = 5.97
Remember that with minute of angle sight adjustments, the farthest you'll be off at 100 yards is 1/2 inch. If your group center is within 1/2 inch of your point of aim, you're good to go.
I've been seeing a lot of folks arguing back and forth about the best way to get a 300 meter "zero" with their AR15 (or other firearms) at 25 meters.
First off, I define a zero as being one of the possibly two points where the bullet's trajectory and the sight line coincide. If I want to zero at 300 meters then I get my butt out to 300 meters and sight-in. If my range is limited, say to 100 yards, but I still want a 300 meter zero then I'll adjust my 100 yard point of impact so it's above my aiming point. But I call that "sighting-in" or a "sight-in" because it's an estimate- possibly an accurate estimate- rather than a true zero. If I've previously zeroed at 300 meters and then fired a group at 100 yards to get my exact hold-under, then it should be just about spot-on.
The problems really occur when people try to sight-in at extremely close ranges such as 25 meters. Trying to adjust your sights to zero at the range where the bullet trajectory first crosses the sight line (later it will fall past the sight line at a longer range) will only work if your trajectory exactly matches the ideal as outlined in whatever manual you may be reading or calculated by whatever program you may be using. Variations in atmospheric conditions, muzzle velocity, ballistic coefficient (which can change with the same bullet design out of different rifles), and even how the shooter allows the rifle to recoil will change the ballistic path. Tiny errors at 25 meters will magnify greatly at longer ranges.
Close range sight-ins can also mask shooter marksmanship errors. If you pull your shots a tiny bit to the right, at 25 meters you will be able to adjust your sights to compensate for this. At 300 meters the error will grow beyond your weapons sight adjustment range and you'll wonder why you're on at 25 but off the paper at 300. Notice that the reverse is not true; small errors at 300 meters will be vanishingly tiny at 25.
So, I'll give you the method I use to sight-in a weapon and then give you some close-range field expedient sight-in estimates for the AR15 series.
The first thing I do is inspect the sights on the weapon. Unless I'm working on a previously zeroed weapon I will return the sights- optical or mechanical- to their mechanical centers and check their adjustment functioning. I’ll also check the mounts to make sure everything is tightened properly.
Next, I will check for any gross visible misalignment between the sights and the bore and correct that with machining, shims, whatever.
If I have an optical collimator, I'll place that on the weapon and check to see if the sights are parallel to the bore. If there's a major adjustment necessary I'll fix the problem. If they're pretty close, I'll adjust the sights to bring them parallel to the bore. I prefer the collimators that have a grid instead of just an "X" so I can check to be sure the sights adjust in the direction and increments they're supposed to. For example, I ran into a number of scopes made overseas that were marked for quarter inch adjustments but were built to metric standards so really gave three clicks to the inch at 100 yards; not a problem but could give a bit of a surprise on the range. I also firmly tap scopes while they're on the collimator to check for a loose reticle.
Next, it's off to the range. I always work off of a solid bench with a firm front and rear bag. If I'm just checking an already established zero, I'd prefer to work from the prone rather than off something unstable like the hood of a car or a wobbly bench. (Note on car hoods: I've seen several windshields cracked or shattered by the muzzle blast of centerfire rifles, not to mention damage to the paint.) Large bore rifles- anything above .300 magnum for me- or powerful spring airguns should be fired with a fairly firm grip on the forearm and the back of the support hand bedded in the sandbag or the recoil off the bag will make your point of impact radically different from what you'll get in the field from standard positions. A tight sling may also change your point of impact. AR15s are notoriously bad about changing point of impact with minor changes in pressure on the forearm so be aware of that. Also, if your rifle has a sling swivel stud on the forearm, make sure the stud is far enough forward of the sandbag that it won't run into the bag under recoil and make your shot go high.
Before I start shooting I will double check my bore-sight by physically bore-sighting on the most distant object I can find; some weapon mechanisms make this impractical. I sight through the bore at a distant point object- in my back yard I use the radio towers on a nearby mountain. Make sure that the aiming point, the muzzle, and the breach end of the bore are all perfectly concentric. Use the sandbags to lock the rifle in this position. Without disturbing the rifle move up to the sights and check to see if they're on target. If you used an optical collimator don't be surprised when the sights are off. I always had much better results by bore-sighting on the range which is why I try to do it if possible. Check back through the bore to make sure the weapon hasn't moved and make any necessary sight adjustments. Repeat the boresight procedure until the sights and bore are parallel.
If you're really brave you can start your sight-in at 100 yards. I only had a couple of rifles that were off the paper after a field bore-sight but one of them was way off. Just remember that when bore and sights are parallel the point of impact is going to be quite low at 100 yards (or whatever range you choose). Unless ammunition is at a premium I prefer to start at 25 yards.
If I'm relatively confident in the weapon and sights I will shoot one shot and make adjustments. With an unknown weapon or if I'm just out of practice I shoot groups of at least three to five shots before making any adjustments. I zero the weapon at 25 yards. With weapons with a very high sight line such as see-through scope mounts or AR15s I will adjust my point of impact about an inch low at 25 yards so it won't be too high at 100. When I get a good 25 yard zero I move back to the next range. I personally use a 50 yard zero with pistols, slug guns, and most of my .22 rimfires. For my other rifles I will move back to 100 yards.
I will zero my rifles at 100 yards and then make any adjustments I feel necessary for longer ranges. An inch and a half to two inches high at 100 will usually give you a pretty good 200 yard zero. If you're limited to 100 yards this will have to suffice, otherwise I prefer to move out to whatever long range I want to zero at and get a true zero.
Here are the close range zeros for the AR15. Remember that these are estimates- there's no substitute for zeroing your own rifle at the actual range. The military's 25 meter sight-in used to be considered field-expedients where a longer range was unavailable.
The Army method for the old style A1 sight was to flip up the long range sight and zero at exactly 25 meters (not yards). When the short range sight is flipped back it will supposedly be zeroed for 250 meters.
The Marine Corps has gone through several methods for obtaining a battle sight zero with the A2 sight at close range. Their first method was to set the rear sight at 300 meters plus one click and then zero at 25 meters using the front sight to make elevation changes. When the sight is clicked back to 300 it's supposed to be spot on. Somebody at AR15.com ran the numbers on his computer and found that this was incorrect. This close range sight-in actually gets you a 400 meter zero. The Marines discovered the same thing and later changed to a 30 meter (36 yards) zero with the 300 yard setting.
According to the current manuals a 25 meter near zero equals a 375 meter far zero, 42 meters equals 250 meters, and 36 meters equals 300 meters.
Here are the number I ran up on my computer. I found several different ballistic coefficients for both the M855 (62 grain) and the M193 (55 grain) so I used an average. My numbers are pretty close to the military's. I also adjusted for my higher elevation- 7500 feet. I ran sets of numbers for both cartridges and in yards and meters. I ran numbers for all the standard long range zeros as well as a point-blank zero that doesn't allow for the bullet to rise more than 2.5 inches above the line of sight. The computer spits out numbers to two decimal places which is ridiculous for sights that only make MOA or half minute adjustments but it won't hurt anything as long as I don't try to make adjustments to thousandths of an inch.
These numbers are all how many inches above your point of aim the bullet should impact at 100 yards to get the long range zero listed.
For the M855 or SS109 round-
200 yards = 1.15 inches
200 meters = 1.56
239 yds (+2.5 inches to -2.5 inches point blank) = 1.98
250 yds = 2.24
250 m = 2.73
300 yds = 3.53
300 m = 4.38
375 yds (for the A1 long range aperture) = 5.77
375 m = 6.81
For the M193 round-
200 yds = 0.89
200 m = 1.25
250 yds (+2.5 inches to -2.5 inches point blank) = 1.85
250 m = 2.29
300 yds = 3.00
300 m = 3.76
375 yds = 5.03
375 m = 5.97
Remember that with minute of angle sight adjustments, the farthest you'll be off at 100 yards is 1/2 inch. If your group center is within 1/2 inch of your point of aim, you're good to go.
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