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Centerfire Rifles - Semiautomatic or Gas Operated Centerfire rifles, carbines and other gas operated rifles.

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  #1  
Old 11-10-2009, 9:16 PM
Bucky G Bucky G is offline
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Default sighting in red dot for 5.56

how high or low do I sight in a red dot at 25 yards to make zero at 100 yards?

it's a really 5.45 but I figure the ballistics are close enough to the 5.56 and you guys prolly have more experience with that caliber
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Old 11-10-2009, 9:21 PM
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1.5" to 2" low at 25 yards.
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  #3  
Old 11-10-2009, 9:58 PM
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thanks!
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Old 11-10-2009, 10:07 PM
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Quote:
Originally Posted by Sheepdog1968 View Post
1.5" to 2" low at 25 yards.
What does that mean ?
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Old 11-10-2009, 10:43 PM
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You can try using the PDF target technique made available in this thread. I found it quite useful.
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Old 11-10-2009, 10:45 PM
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It means your point of impact will be 1.5-2" lower than your point of aim.

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Originally Posted by 1988 View Post
What does that mean ?
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  #7  
Old 11-10-2009, 11:06 PM
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Quote:
Originally Posted by 1988 View Post
What does that mean ?
The bullet will drop in free air about 3ft for every half second it is airborne.
The aerodynamic shape and velocity of the bullet will determine how far it will drop over a given distance.

Mythbusters recently covered this using a .45
Basically, if you use a laser boresighter to "zero" and fire the gun with the barrel 3ft above the ground, the bullet will impact and slide across the ground approximately 450-475ft downrange.
That's based on the velocity of the .45.
A 5.56 is moving much faster, so it will travel a longer distance in that same half second.

Adjusting the red dot 1.5 - 2" low at 25 yards puts it at approximately the correct location to allow for drop over 100 yards.

That's why the AK47, and some other guns, have numbers and a sliding barrel on their rear site. Sliding the barrel to the larger numbers (longer range) raises the rear site, which forces you to aim high and compensate for bullet drop.
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Old 11-11-2009, 12:24 AM
1988 1988 is offline
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Quote:
Originally Posted by Arteeel View Post
It means your point of impact will be 1.5-2" lower than your point of aim.
Quote:
Originally Posted by Cokebottle View Post
The bullet will drop in free air about 3ft for every half second it is airborne.
The aerodynamic shape and velocity of the bullet will determine how far it will drop over a given distance.

Mythbusters recently covered this using a .45
Basically, if you use a laser boresighter to "zero" and fire the gun with the barrel 3ft above the ground, the bullet will impact and slide across the ground approximately 450-475ft downrange.
That's based on the velocity of the .45.
A 5.56 is moving much faster, so it will travel a longer distance in that same half second.

Adjusting the red dot 1.5 - 2" low at 25 yards puts it at approximately the correct location to allow for drop over 100 yards.

That's why the AK47, and some other guns, have numbers and a sliding barrel on their rear site. Sliding the barrel to the larger numbers (longer range) raises the rear site, which forces you to aim high and compensate for bullet drop.


Why do we want the POI at 1.5-2" lower than POA ?

Do we know the configuration of the rifle ?

Do we know the muzzle velocity ?

Do we know the BC of the bullet ?

Last edited by 1988; 11-11-2009 at 12:40 AM..
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Old 11-11-2009, 12:27 AM
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Quote:
Originally Posted by 1988 View Post
Why do we want the POI at 1.5-2" lower than POA ?

Do we know the configuration of the rifle ?

Do we know the bullet weight being use ?

Do we know the muzzle velocity ?
In the 25-100 yard range, the main issue is the optics not being on the barrel's centerline.

Beyond 100 yards is when drop becomes a major consideration.

I know... that's in conflict with what I said before... I had the same question as you until I thought it through.
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Quote:
Originally Posted by dantodd View Post
A just gov't will not be overthrown by force or violence because the people have no incentive to overthrow a just gov't. If a minority of people attempt such an insurrection to grab power and enslave the people, the RKBA is our insurance against their success.
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  #10  
Old 11-11-2009, 12:44 AM
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Quote:
Originally Posted by Cokebottle View Post
In the 25-100 yard range, the main issue is the optics not being on the barrel's centerline.

Beyond 100 yards is when drop becomes a major consideration.

I know... that's in conflict with what I said before... I had the same question as you until I thought it through.
We're shooting crab if we don't know the distance between the center of the bore and the line of aim, the BC, and the muzzle velocity.
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