Zero on an AR15

I have recommended a 300 meter zero like the USMC teaches. But a reader asked me - how do you zero for 300m if you only have a 100-yard range?

GREAT QUESTION!

Note: All measurements have been verified using a ballistic calculator.
For M855 or similar:
- the ballistic coefficient is .304
- the velocity from a 20" barrel is 3100 fps (subtract 50 fps for each inch shorter the barrel)
- bullet weight is 62 grains

For M193 or similar:
- the ballistic coefficient is .243
- the velocity from a 20" barrel is 3200 fps (subtract 50 fps for each inch shorter)
- bullet weight is 55 grains

Using RIBZ
Know and understand that I believe in having a Revised, Improved Battlesight Zero (RIBZ) on all AR15 iron sights. If your iron sights can be adjusted like that with either a 6/3 or 8/3 dial, then simply set it to the 100-yard setting and zero as normal. After the rifle is zeroed, then set it to 8/3 and forget it.

No elevation
However, not all rear iron sights have an elevation dial. If your rear sight is fixed with regards to elevation, then here is an easy way to obtain a 300m zero (or really darn close).
This is common for most flip-up iron sights such as Magpul, Troy, or similar. 

1. Sight in at 25 yards to be on the button.
2. Move your target to 100 yards and fine-tune the zero such that your bullet impacts about 3.25 inches high from your exact point of aim.
(Alternately, sight in on the button at 100 yards, then rotate the front sight post "up" 3 clicks)

No elevation II
A 300m zero is fine for practical purposes, but it is not the most popular zero with flip up irons. The most popular zero is a 200m zero (which, incidentally, is really close to being dead on at 50 yards, too). I am not at all against this zero - it work very well.

1. Sight in at 25 yards to be on the button.
2. Move your target to 100 yards and fine tune such that the point of impact is 1 inch high.
3. Remember that at 300 yards, you will need to hold over about 7 inches (one dot over on most red-dot optics).

Other elevation settings
Suppose you have a MATECH or Knight's adjustable elevation backup iron sight. Both sights have settings from 200 yards to 600 yards.

1. Sight in at 25 yards on the 300m setting to be on the button.
2. Move your target to 100 yards and fine tune the zero such that the bullet impacts 1 inch high when set on the 200m setting (or 3.25 inches high on the 300m setting).
3. Set the sight to either 200 or 300 and forget it.


.