ZERO: The range at which a firearm will hit the point of aim. In shooting, the sight setting at which point of impact and given point of aim coincide at a given range.
ZEROING: The technique of setting your sights so that a shot called good, (undisturbed perfect sight alignment and minimum arc of movement) will hit the center of the target on an ideal day with no wind.
For additional information on Zeroing see the detail
Some Thoughts On Zeroing
Properly zeroing one's firearms is of paramount importance. As received from the factory, firearms have only a rudimentary "zero" if any, which may or may not correspond to where the bullet will actually impact when fired by a given individual. Thus it is necessary to establish a proper zero with your firearm using your particular ammunition. In addition, you have to zero your firearm yourself because each individual holds a firearm differently which can greatly affect where the bullet strikes.
The first step in achieving a zero is to get the strike of the bullet close to where you want it. In order to do this expeditiously you will need to know the amount of movement provide by your sighting device per graduation of its dial or mechanical "click." This information is available from your sight's manufacturer. Keep in mind that most devices are calibrated to give a specific movement at 100 yards and that if you shoot at a shorter range that the movement is proportionally reduced. The chart below shows what happens.
Thus if you are shooting at 25 yards with a sight graduated in 1" marks or "mechanical "clicks" for 100 yd and you are 2 inches off of your point of aim at 25 yd you will need to move your sight's adjustment eight (8) graduations or clicks.
For fixed sights the problem is a little different since you need to either physically move the rear sight for deflection or alter the height of either the front or rear sight for elevation. The formula to determine the amount of adjustment is:
Note that to move a bullets strike horizontally move the rear sight in the direction you need to go. To adjust elevation either lower the front sight or raise the rear sight to raise the bullets impact, or raise the front sight or lower the rear sight to lower the bullet's impact. As an example if your sight radius is 5" and you need to move the bullet's strike 3" higher at 25 yd (75 feet):
Thus you need to lower the front sight (or raise the rear sight) by .016 inches.
Getting on Paper
Now that you know how to make the adjustments you need to first get the bullet's strike close to where you want it and to then refine your zero. You can start in one of several ways.
1) Firing from a supported position on a bench rest, CAREFULLY fire one shot at a distinct mark on your backstop. (With rifles do this at the longest practical range. Have an observer note the bullets strike in relation to that mark. (If you have been a very good little boy or girl you may be right on, but don't count on it.)
If necessary, make an adjustment--see the above instructions--for deflection (side to side) to bring the bullet's strike to the horizontal point of aim. CAREFULLY fire a second shot at your mark. Have your observer note the bullets strike in relation to that mark and make an adjustment for elevation (vertical) to bring the bullet's strike to the vertical point of aim. You should now be close enough to work on your final zero on a paper target.
If you don't have anyone to observe for you it may be necessary to fire at a paper target with a distinct aim point and a grid (possibly at a closer range--say 25 yd) and adjust the sights to center the shot.
Handguns should be fired either from a sand bagged rest or from the braced sitting position.
2) Use an optical collimator such as the one made by Bushnell. A collimator is simply an optical device which consists of an optical "head" and an adjustable rod which fits the bore of the firearm tightly and which creates an image of a target at "infinity" that you can use to adjust your sights. Follow the manufacturer's instructions for use, and with the collimator in place adjust your sights to center them on the collimator's reticle. These devices are primarily used with rifles.
3) With a firearm that allows a direct view through the bore you can carefully sandbag the firearm in place, look through the bore, and then (without moving the firearm) adjust the sights to intersect the center of the bore's view.
Keep in mind that you have to shoot VERY carefully in the above steps, without jerking the trigger or disturbing the sights. If you jerk the trigger you'll never be able to obtain a good zero.
Once you have gotten your shots roughly centered on a target it is time to refine the point of impact to give you the optimum zero.
Please note that I have not rounded velocity and drop figures to eliminate meaningless precision as I have done on my other articles. It simply takes too much time to edit the tables. Keep in mind that knowing velocity within 10 fps and drop within a 1/10th inch under 100 yards and within 1 inch past that range is precise enough for all but techno-weenies.
Too many shooters zero their rifles (for that matter, all their firearms) at too short a distance and thus loose the advantages of a more useful trajectory. For the most efficient use of trajectory you want to zero your firearm for the farthest distance over which the actual point of impact vs. point of aim will be within the critical (vital) zone of your expected target. That is, the maximum ordinate or maximum height of the bullet's trajectory will not be greater than one half of the intended target's vital zone.
If you zero for too close a range you make hitting at greater distances much more difficult. While you may say, "but I'll never shoot anything at a greater distance" one never knows when that trophy buck or a target of opportunity may show up way over there. Since, with a proper zero one doesn't have to really worry about hold over (or under) until the ranges approach 300 yards away, why not take advantage of what you have, even if all you do is hunt in the thick brush of New England.
Your critical zone size will vary depending on your intended target but ± 3 - 4 inches is a good compromise for most uses. The table below gives the diameter of the vital zone and suggested maximum ordinate for some common game.
The table below, based upon the 7.62 mm NATO GI M80 ball round (G7 = .195) and shows the effect of different zeroing ranges. The effect of differing zero ranges will still hold true for other bullet weights and types.
You can see that a good zeroing range for the .308 / 150 gr is somewhere between 200 and 250 yards depending on the individual projectile if we accept a vital zone diameter of between 6 and 8 inches. Interestingly this also works out very closely for other bullet weights and cartridges with a muzzle velocity in the range of 2400 to 2900 f/s. If we zero our rifles to be between 2¾ and 3¼ inches high at 100 yards we will pretty much be in the ball park no matter what bullet you use.
In the interest of completeness lets look at the effect of ballistic coefficient on optimum zero. In the table below I have taken the worst, average, and best case published ballistic coefficients for standard commercial .30 caliber 150 gr pointed (flat base and boat tail) bullets at a muzzle velocity of 2750 f/s and assumed a maximum ordinate of 3 inches.
Notice that if one zeros to be about 2.75 inches or a little higher with any 150 gr pointed bullet that you will have pretty much an optimum zero assuming a 6 inch vital zone and have a point blank range of 265 yards ±. We could pick nits but for practical field use you'll be just fine. If we choose a different maximum ordinate things still hold up nicely.
If we are using a cartridge with a velocity substantially higher than the above we can use it to get a a longer point blank range by using the same 100 yard point of impact as above. If you want to refine things, this is a great reason to acquire a good ballistics program and by playing with different parameters you can learn a lot. I highly recommend RSI's Shooting Lab. (See my main ballistics page for details.)
Most folks think that the trajectory of the 12 gauge rifled slug is close to that of a mortar. Since they don't think they could hit anything past 25 or 50 yards (which is probably true if they don't have a set of sights on their shotgun) they zero for slugs at 25 yards. Unfortunately, this short zero severely limits the effectiveness of the slug firing shotgun. Surprisingly, a slug's trajectory is quite flat out to about 125 yards. The biggest limitation of the shotgun slug is that penetration and trajectory drop off drastically beyond 125 yards due to velocity loss, so its maximum effective range is probably about 125 yards. (I still wouldn't want to be hit by a slug at 200 yards though!)
Foster Type Rifled Slug (G1 =
While the 100 yard zero appears to be more useful than the 75 yard zero, the fact that most standard riotguns only will group into 8"-10" at 100 yards makes attaining a good 100 yard zero difficult unless sighted in at a shorter range with compensation for the distance. A lot of folks find it easier just to zero them for 2" high at 50 yards (at which distance group size is usually quite good) which gives about an 85 - 90 yard zero which is probably just fine. Using a properly zeroed shotgun and slugs with a good set of sights one can completely control their environment with a 125 yard radius.
By the way, for those of you interested in such things (even though at typical buckshot distances it doesn't matter) the G1 ballistic coefficients for buckshot are approximately: 1 buck - .036; 0 buck - .037; 00 buck - .038; and 000 buck - .042. Just for fun, here is the data on 00 buckshot. A 75 yd "zero" is assumed.
Most folks believe that pistols are only useable at very short ranges and thus zero them at very short ranges. As with rifled slugs in a shotgun, most people believe that pistol bullets have such a curved trajectory that long range hits are next to impossible for other than certain "never miss" gun writers and Uncle Elmer's 600 yard deer. (We'll ignore the silhouette shooters for the time being since that is a specialized activity, usually with optical sights and with equipment that often stretches the definition of "pistol."). While the handgun is primarily designed for use at short ranges (50 yards--and usually much less), don't feel under gunned if your target is at greater ranges (assuming that you know how to shoot) if you have a proper zero. The following table shows the trajectories of typical 9mm 125gr, .357Mag 158gr, and .45ACP 230gr, and .44 Magnum ammunition with a 50 yard zero and .8" sight height. (Note that the .44 Magnum can be usefully zeroed at 100 yd which will give it a maximum ordinate of 2.6" at 56 yards and it will be about 9.8" low at 150 yards.)
Typical Handgun Ammunition Trajectories
Long range defensive or long range hunting use of a pistol is not recommended except in dire straits. However, when the goblin is way out there near Fort Mudge and thinks he's out of harms way, if you hold on the head and carefully squeeeeeze one off you'll easily get a chest hit (and ruin his whole day). I once wowed a couple of MPs by getting 5 solid chest hits with 5 shots on an IPSC "option" silhouette at 100 yards using a Detonics pocket .45 from the braced sitting position. If I can do it so can you!
As far as I know all the information presented above is correct and I have attempted to insure that it is. However, I am not responsible for any errors, omissions, or damages resulting from the use or misuse of this information, nor for you doing something stupid with it. (Don't you hate these disclaimers? So do I, but there are people out there who refuse to be responsible for their own actions and who will sue anybody to make a buck.)
|ZERO POINT: The location of
the center of a burst of a nuclear weapon at the instant of detonation.
The zero point may be in the air, or on or beneath the surface of land or
water, dependent upon the type of burst, and it is thus to be
distinguished from ground zero.
ZK383: The ZK383 is a Czechoslovakian sub-machine gun used during the second world war. It takes a 9mm round from a 30-round box. It has a cyclic rate of 500 and 700rpm and is sighted to 800m with a muzzle velocity of 365 m/s.
ZOUAVE: Pronounced: Zu-'äv from the French, from the Algerian Berber Tribe of ZwAwa of the 1830's 1 : a member of a French infantry unit orig. composed of Algerians wearing a brilliant uniform and conducting a quick spirited drill 2 : Member of a military unit adopting the dress and drill of the Zouaves.
ZSU-23-4: Russian Air Defense Artillery (ADA) system consisting of a four (4) 23mm (.91 inch) radar directed, automatic cannons mounted on tracked and wheeled carriages
ZU-23: The ZU-23 is a Russian 23mm (.91 inch) auto-cannon mounted on Mi-24 Helicopters and Su-25 attack jets. It has a muzzle velocity of 970m/s and can pierce 30mm of armour at 500m. It was designed in the 1940s and has become the standard gun mounted in Russian jet aircraft.
ZONE FIRE: Artillery or mortar fires that are delivered in a constant direction at several quadrant elevations.
ZULU TIME: Same as Universal Time Coordinated (UTC) formerly known as Greenwich Mean Time. Zulu Time or "Z Time" is used to coordinate world wide military operations as the time standard, since "local time" is different in the 24 global time zones.