Forgive my ignorance but I understood that the .223 has very little variance in trajectory from the 25 to 100 yard line.

Yesterday, I attempted to zero my H&K G36C (9" barrel) at the 50 yard line using a new Eotech 511 sight. I was using the Federal M193 white box. I brought it to a nice grouping.

I made no adjustments from the 50 yard groups of about 2" center target. When I went to the 25 yard line, I saw the groups drop about 2 inches and at the 100 yard line the groups were about 4" too high.

I don't know much about this stuff so any help would be appreciated. The temp was about 80 degrees with no wind. I'm at about 4000 ft elevation. Is it the short barrel? Ammo?

I know that I have alot more testing to do so please help steer me in the right direction.

I have my ar-15 (A2) sighted in at 100 yards and have the same problem. The bullet is still climbing and therefore the bullet is low until it gets out to about 100 yards or so. I'm not sure but I think it's because the sights sit so high up on the gun relative to the barrel.

What you describe makes complete sense. The variance you are seeing at the different ranges is directly related to the height above the bore that you sight is mounted. The higher your sights are mounted above the bore axis the greater the variance in your point of impact at ranges closer than and beyond your 50 yard established Zero.

Basically the trajectory of your projectile is an arc. Exactly where on that arc you establish your Zero will determine what happens at closer and more distant ranges. Don’t forget you need to re-establish (or at least check) your Zero for any change in type of ammunition.

A 100 yard/meter zero is always low, other than at 100 yards/meters.  Bullet starts out 2.6" low and the trajectory peaks at 100 with this zero, then drops down.  Pretty useless as a battlesight 'cuz you always have top aim high.

If you zero at 50 yards the bullet still starts out 2.6" low, crosses the line of sight at 50, continues to climb to about 2" above line of sight out around 150 to 170, then drops back down to line of sight about 200 meters.  Not more than 2" from line of sight from just beyond the muzzle to 220 meters.

This trajectory is the basis of [url=http://groups.msn.com/TheMarylandAR15ShootersSite/improvedbattlesightzero.msnw]Improved Battlesight Zero (IBSZ)[/url].  Developed for the M4/M855 but works well enough of rifles.

-- Chuck

I can't comment on your G36C With EO Tech because you didn't tell us how high the optic is off the bore.

This is VERY important when developing a zero for a rifle.

With the AR-15 the irons are 2.6" over the bore's centerline (most optics get mounted at or near this height as well).

With this HOB (Height over bore) using Chuck's IBSZ (mentioned above) and having a 50 yard zero with a 16" barrel you should get rounds that are 1.2" low at 25 yard, dead on at 50 and about 1.6" high at 100 yards (still rising - maxes to 2.0" @ 150y)

Now if I switch to my Compact ACOG (HOB is now 3.5" zero is still at 50y) then I get these results:
-1.6" @ 25 yards, dead on at 50, and 2.5" high at 100 yards (and still rising - maxes at 3.9" @175y)

A short barrel will reduce the MV and change the trajectory somewhat (especially at longer ranges) but there shouldn't be too much difference under 100y (and at 100y it should be lower - not higher than the 16" barrel).

Can you measure the distance from the center of the optic to the center of the barrel?  Also what is your barrel length or MV (if you had a chrono)?

BTW part of this may be the shooter pulling the rifle down wanting to see where the bullet hit - don't laugh I've seen it.

Thanks everyone for the help.

I measured the distance from the barrel to the center of the EOtech sight window and it measures 4 1/4 inches. This seems to correlate to the problem as you all have described.

For 4.25" and M193 out of a 16":
@ 25y 2" low
@ 50 dead on
@100y 3.2" high.

Now these figures are for the center point of the group.  If you ammo groups say 3" @ 100 yard you'll have rounds striking from 1.5" high to 4.5" high.

HTH

That stuff about the 25 yd zero being close to the 100 yd zero is a hold-over from conventional bolt acttion guns which have sight lines very close down to the bore line. In this case, the round will cross up through the line of sight at 25 yds, peak in its arc, and fall back down through the line of sight at about 100 yds.
However, the line of sight on your AR (irons or optics) is way above the bore line. This means that if the bullet crosses the line of sight up-bound at 25 yds, it is doing so at a ferocious up-angle. Therefore, by the time the bullet gets to 100 yds, it is reaching for the ionosphere.
I would suggest a 100 yd. zero. Why? Close in CQB you are going to have to aim high in any event. To prove this to yourself, stick the muzzle of your AR right into the intended point of impact of a target. What do you see when you look through the sights? See? At least with a 100 yd zero, if you have a problem at 100 yards, you came aim at the chest and hit it (rather than having to aim at his pee-pee in order to make a chest shot). And if the problem is at 200 yards, you only have to aim about 1.5" high.
Good luck. Good shooting.

You just convinced me to change my sight in from 100 to 50 yards. Good thinking