Zeroing Problems

[NemoCaufield]

Question on zeroing a Bushmaster shorty ak. I followed the direction in my manual and on ar15.com
I left the front post alone and centered windage and put the rear sight elevation on 8/3 plus one click up using the smaller aperature using a 25 meter range. I fired a couple of shots adjusted windage and I had 1" groups. This was with all the above set. I thought that zeroing a 25 meters would set up my rifle for a 300 meter battle sight. So I then placed the target again at 25 meters. I clicked the rear sight elevation to 8/3 and put up the larger aperture meant for short range shooting from 0-200 meters. Then my shots for the 25 meters were way off. What did I do wrong? I thought that by doing the 25 meter zeroing my rifle would be good for all ranges? what happened?

 

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Nemo,

The trajectory of the bullet is always a curve that drops from the muzzle to the ground, when the bore axis is parallel to the ground.

IF the line of sight (iron sights or scopes) was parallel with the bore axis, you'd be dead-on, immediately in front of the muzzle.

Since the line of sight is offset from (above) the bore axis, the line of sight and trajectory must INTERSECT in order to aim at a distant target.

It is possible to adjust the line of sight relative to the bore axis to create only one pont of intersection (TANGENT) along the path of the trajectory curve of the bullet. However, this limits the utility of the sighting system to just that distance.

It is also possible to adjust the line of sight relative to bore axis to create two intersects with the trajectory curve of the bullet, a near intersect & a far intersect. This is the case of the 3/8 elevation setting for the A2-style sight system.

Zeroing at 25M requires a special target or using a specified offset aiming point, because you're actually setting the dead-on points for 300 & 800 meters.

Zeroing at 25 meters requires the adjustment of the front sight when the rear drum is set at 3/8 (small aperture) & windage is set (hopefully at 0). Remember the aiming point and striking point are not the same for the 25M target!

Once this process is done correctly, the full-sized AR, shooting GI ball ammo should have a battle-sight zero of 300M.

NOTE the shorty barrel has a shorter sight radius than the full-sized rifles (unless you have a "dissipator" barrel which retains the original sight radius). This means that each sight adjustment increment is effectively larger (makes a greater change) than on the full size rifle. Although the change may not be that apparent between 25-100M, it will be significant at 300 & 800M. Consequently, you will need to zero & verify the battle-sight with the specific ammo which you plan to use in your specific barrel. You may find that you'll need to go to NM (national match) type sights (smaller increments of adjustment) to compensate for your ammo & barrel length combination, then settle for adjusting to a 300M battlesight, and calculate/verify the correct "holdover" at 800M. This latter is because the shorter sight radius effective moves the "far" intersect of the line of sight further down the bullet trajectory, thereby making the intersect greater than the 800M distance.

The advantage of optical sights on the shorty is they bypass this sight radius voodoo & the error compensation calculations. You'd be back to adjusting just for the cartridge ballistics & windage.

Good Luck!

[This message has been edited by Mykl (edited May 31, 2000).]