POI - What Am I Missing ?

AlanInSilverSpring · Nov 2, 2017

I'm thoroughly befuddled.

Took my ar15 to Heritage and adjusted the scope so it was dead on at 25 yards.

Couple weeks later took it to Delmarva and shooting at 50 yards everything was in about a 3 inch circle, which I think was pretty good considering it's a run of the mill rifle, low end scope, my limited skills, and shooting cheap a$$ Wolf ammo.

Then we (me and my friend) tried it out at 100 yards and every single shot was maybe 8 inches high. Something was definitely wrong so we went back to the 50 yard target but it shot just like before.

Doesn't make any sense to me that we could both be off that much so What am I missing (not understanding) ?

Half-cocked · Nov 2, 2017

Zero at 50, not 25.

All your problems will disappear.

Ranchero50 · Nov 2, 2017

Bullet trajectory determines where the POI is vertically in relation to the POA. Horizontal is wind. 10 yards should be @ 2" low POI vs. POA due to sight height vs. barrel height. Try to find the trajectory of the rounds you were using and google the arc.

Qbeam · Nov 2, 2017

Ballistics, see the chart in the thread:

http://www.m4carbine.net/showthread.php?54081-25-yard-range-zero

Q

AlanInSilverSpring · Nov 2, 2017

Thanks for the info. I never would have guessed the bullet would rise that much.
And never would have guessed that a 50yd zero would yield such better results from 25-300 yards

At least that chart confirms what I observed. I didn't measure but guessing a 6-8 inch rise at 100 yards is right on.

Melnic · Nov 2, 2017

Considering how high above the bore axis the scope would be, what you describe sounds about right.
to get it to shoot center at 25, the scope optics (not the scope but the optics) has to be pointed downward (adjusted by the scope adjustment)
So the AR which has a decently flat trajectory within 100yds (if you had the barrel perfectly horizontal) at 50, it will be high, then at 100 it will be higher

http://www.shooterscalculator.com/ballistic-trajectory-chart.php

select 223 remington
change zero to 25 yards
Adjust chart range to 100 yards
Set scope height (center of bore to center of scope) to what you have (I'd guestimate 2")
Your going to be at 4" off the bat.

If you zero at 50 it will still be pretty close (on paper) at 200 yards.

AlanInSilverSpring · Nov 2, 2017

More great info. Thanks. Love that calculator !

Buckeye_Nut · Nov 2, 2017

AlanInSilverSpring said:
Thanks for the info. I never would have guessed the bullet would rise that much.
And never would have guessed that a 50yd zero would yield such better results from 25-300 yards

At least that chart confirms what I observed. I didn't measure but guessing a 6-8 inch rise at 100 yards is right on.

You do realize now that the bullet doesn't actually "rise"?

AlBeight · Nov 2, 2017

AlanInSilverSpring said:
I'm thoroughly befuddled.

Took my ar15 to Heritage and adjusted the scope so it was dead on at 25 yards.

Couple weeks later took it to Delmarva and shooting at 50 yards everything was in about a 3 inch circle, which I think was pretty good considering it's a run of the mill rifle, low end scope, my limited skills, and shooting cheap a$$ Wolf ammo.

Then we (me and my friend) tried it out at 100 yards and every single shot was maybe 8 inches high. Something was definitely wrong so we went back to the 50 yard target but it shot just like before.

Doesn't make any sense to me that we could both be off that much so What am I missing (not understanding) ?

Do you have the scope on a 20 MOA or other type of sloped mount? That’ll mess with your closer ranges because the scope isn’t really level. My RPR confounded me until I got this thru my skull.

AlanInSilverSpring · Nov 2, 2017

Buckeye_Nut said:
You do realize now that the bullet doesn't actually "rise"?

Yes I do ! I knew I didn't have magic bullets but just didn't know how else to say it.

I knew it had to be something pretty basic but you don't know what you don't know, which is why I asked the question. And I thank the people that gave the answer without being condescending.

Seems so simple now....... zeroed at 25 yards scope is dramatically pointed down, so when looking out to 100yds scope is still pointing down which makes barrel pointed up (relative to poa).

Biggfoot44 · Nov 3, 2017

As everyone has said, your rifle performed exactly as expected for how you had it sighted.

The reason why is the basic design of the rifle.

AR Platform rifles have straight line buttstocks. Consequently the sights / sighting plane is a metric crap load above the bore centerline , creating the exagerated "rise" of the bullet to get up to line of sight, and the subsequent hold overs and hold unders .

By comparison normal rifles have some drop in the buttstock, and the sights/ sighting plane are much closer to the bore . Consequently less radical ups & downs of poi vs poa .

For illiustration only , actual measurements will vary . Default value for " scope height" for normal rifles ( typical charts, and default setings most ballistic programs ) = 1.5in. .

Common default value for AR = 3.5 in.

POI - What Am I Missing ?

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