AR sighting question

[ChCx2744]

Hello. I have a Bushmaster XM15 E2S with a removable carry handle. When I first bought it, I zeroed it at 25 yards then again at 50 yards. When I zeroed it, I did NOT click the rear elevation knob "down" 2 clicks to "Z" (zero). I left the rear elevation at 6/3 (300 meter battle sight), small aperture. From there, I only adjusted the front sight post and the rear windage knob left/right. My question is, how come when I shoot it out to 50-100 yards, it pretty much hits where I aim, but anywhere 25 yards and under it shoots a little low? How is that possible? It shoots on at longer distance, but low at shorter distance...That seems like it defies the laws of physics...Did I mess up during the sighting process or what? I obviously didn't follow the directions in my manual, because I was supposed to put the rear elevation to "Z" first before doing any of the stuff I did, but I zeroed it like that...Someone please explain

 

[slabuda]

Because the barrel is quite a bit below the sight plane. Its more exaggerated the closer you are to the tgt.

The round is still "rising" on an upward trajectory till it intersects @ 100 yds.


Turning down to "Z" at 25 yds eliminates this and puts POA @ POI. Then going to 6/3 returns it to battle sight zero.
Dont worry about it what you experienced is normal. Sighting in @ 25 is just to get you on paper and in the ball park so you can fine tune it at your true zero range

 

[Powerglide]

What Slab said

 

[ChCx2744]

So from now on, if I use the "Z" instead of "6/3" for distances 25 yards and UNDER, I should be hitting POA?

 

[Powerglide]

Still, the point of eyesight is higher for the first few yards, no matter where you set your sight.

 

[Powerglide]

The bullet from an average rifle usually crosses the sight path around/near 20 meters, give or take.until then, it's rising.

 

[ChCx2744]

Oh how the physics of rifle ballistics are confusing

So on the rear elevation knob, do you guys use the "Z" zero sight or the 300 meter battle sight for overall use?

 

[Kwanger]

Personally I use a 200 yd zero. If you don't have 200 yds to play with, I'd zero at 50 as it gets you very close at 200 yds. Why 200 yd and not 300? I rarely shoot much past 300 and the 200 yd zero is flatter shooting at closer ranges.

 

[Sky]

25yards = 300 zero

50yards = 200 zero

I have the same AR and got on paper at 25 then did the 50 yard fine tune. With that weapon I plan on 250yards or less type shooting. Will I take a longer shoot? Yes.
Good little gun,yes.

 

[Mot45acp]

I thought 50 yards = 250 yards. Is it different for rifle and carbine?

 

[Sky]

The 25 = 300 and the 50 = 200 seems to work for me. There are some ballistic guys around here that know mucho more than me.

This simple formula for me works and if you get on paper at 25 you can either stay there and get her squeezed in close or move out to 50. Using this method (for me) requires very little adjustment for the 300 or 200 yard final zero.

 

[Quentin]

There is a difference between rifle and carbine. To get a 300 meter battlesight zero at 25 meters with a 20" barrel you click to the "Z" as the Bushmaster manual stated. Return it to 6/3 and you should have a 300 meter zero. If you're zeroing a 14.5" carbine, leave the elevation wheel at 6/3 when zeroing at 25 meters as the bullet path intersects POA at 25 and 300 meters.

You might want to look at the Santose IBZ and reindex the elevation wheel before zeroing so you can click below 6/3 and have a 50 yard zero that will keep POA and POI within 2.5" from 0 to 225 meters, depending on ammo and barrel length. If you click back to 6/3 you have the standard 300m BZ (and standard ranging of the elevation wheel above 6/3).

 

[Kwanger]

I thought 50 yards = 250 yards. Is it different for rifle and carbine?

That's only one variable...there are tons of others - bullet weight, velocity, ballistics coefficient, temperature, humidity, etc etc etc. Then there's the confusion between meters and yards to add to that mix....they are not interchangeable units of measure, but often get used like that in laymans discussion.

Sky is right with his rule of thumb 50 yards = 200 yards using a 62gr bullet. With a 55gr bullet moving a little fast it is more like 50 yds=220 yds......but absolutely nothing is set in stone; the only way to get a true zero further out is to zero further out.....a close zero is really just an "it will do" one.

 

[benEzra]

I zero mine at 50 yards, and it's close enough to the line of sight to not have to bother with holdover out to 250 or so (far zero is around 200).

If you're zeroed at 50 yards, the bullet starts out about two and a half inches low at 0 yards (that's the distance from the bore axis to the top of the front sight) and is launched at a slight upward angle, where it crosses the line of sight at 50 yards (the near zero), stays above the line of sight for a while, then starts to fall and crosses the line of sight again around 200 yards (the far zero) and keeps falling, so the further you go past 200, the lower the bullet will be.