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55g vs Heavier Bullets...POA-POI Differences?

Discussion in 'Black Rifle Forum' started by Agent6-3/8, Dec 20, 2010.


  1. Agent6-3/8

    Agent6-3/8
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    While my AR is not, my primary HD weapon, I'd still like to keep a mag or two on hand of good defensive ammo on hand. (may also end up using it on coyotes at some point) The general consensus seems to be that the heavier bullets are the most effective fight stoppers.

    At this point I'm shooting 55g loads because its what is most readily available in my area. My question is how large of a POI varaition is there once one start stepping up to the heavier bullet weights? I'm sure at nominal SD ranges, its not s concern, but I'm OCD about things...;)
     

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  2. Travclem

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    in my experience you can count on about 1.5" more drop at 100yds with 75gr than with 55gr. I have a 700p and at 100yds the federal TRU 55gr, Hornady 75gr TAP and Winchester ranger 64grsp all grouped within a 2" circle. I shot 5rds of each with the same poa. My AR tested similar to this but I added xm193 to the equation with it.

    For all intensive purposes, there's negligible difference in poi for these rounds. (inside of 100yds that is)
     

  3. TimP

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    like anything, it will depend on the barrel and ammo combo.

    If the heavier bullets produce the same barrel harmonics as the 55gr, then you should only have to deal with a slight elevation difference as the heavier bullets wont rise as quickly.

    If the harmonics are vastly different, you may toss rounds as far as 2-4 inches off target a 100.

    Simple answer.... you just have to try it and see.


    Here is an example of mine. In the target below you will see three groups. Top left is Blackhills 6.5 Grendel 123gr lapua scenar, middle group is Alexander Arms 123gr Lapua Scenar, and right group is Wolf 120gr MPT.

    The gun was sighted in with the Alexander Arms ammo. As you can see, even though the black hills ( on the top left ) was the same weight, the groups were horrible at 4.25moa, compared to the Alexander arms that grouped .588 inch for the same weight ammo.

    So it really all depends on what your gun likes.

    [​IMG]
     
    #3 TimP, Dec 20, 2010
    Last edited: Dec 20, 2010
  4. glock22357

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    Unless you are shooting you AR for maximum accuracy at ranges past 150M, I would doubt that you would notice a POA/POI difference based on bullet weight. Even if you did, it's simply a matter of sighting in your rifle for the load you choose to use and distance you choose to shoot at.
     
  5. glock22357

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    Bullets never rise, it's a matter of physics laws.
     
  6. TimP

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    they have to rise to the line of sight on an optic or iron sights. Whether you want to call it "rising", or moving towards the intersection point for the point of impact and aiming point with the sights.

    it is true, a bullet will never gain altitude if it is shot horizontally from the ground, although if you are shooting at extended distances, you will actually have to shoot "up" causing the bullet to gain elevation, but not on its on.
     
    #6 TimP, Dec 20, 2010
    Last edited: Dec 20, 2010
  7. Boxerglocker

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    :popcorn: Tagged this thread, Newbie wanting to hear and learn.

    One question though, what distance should one zero thier new rifle with a red dot for typical urban combat distances? From what I have read so far, 25 yards then out to 100 for both iron sights and the optic, is what I'm thinking :dunno:
     
  8. TimP

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    It depends on what optics you are using.

    If you are using something like a Burris XTR, or ACOG, it is designed for a 100 yard zero in order for the reticles bullet drop features to work correctly.

    If you are using a red dot like an Aimpoint, Eotech, another optic without a calibrated reticle or iron sights, I prefer a 50/200 yard zero. It keeps you within 1.5 inches high or low out to 200 yards, and only 7.25 inch low at 300.

    In my opinion the 25 yard zero is a very poor choice. Zeroed at 25yards, you are roughly 4.5 inches high at 100, 6.25-inch high at 150, 6.67-inch high at 200, 5.5inch high at 250, 2.8-inch high at 300, and finally back at point of aim at around 335 yards.
     
  9. plasticslap

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    Where do I start?....

    Bullets never rise. This is a common misconception due to the large population of people that don't fully understand how their rifle sighting system works. When you look down the irons or through a scope, you are looking along a straight path. The barrel however, is angled in your hands in such a way that the bullet will leave the bore headed skywards at an angle that allows it to fall on the intended target at the end of the scope line of sight.

    If the barrel were held at exactly 0 degrees (horizontal) the round would leave and show the effects of the only two forces acting on it, Gravity and horizontal acceleration. Neither of these acts on a bullet to elevate it against gravity, which is independent of the object's motion along the flight path. Newton explained this by proving that forces acting on an object have no effects on the object's motion that are perpendicular to the force.

    [​IMG]



    Hope that helps.
     
    #9 plasticslap, Dec 20, 2010
    Last edited: Dec 20, 2010
  10. Bushflyr

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    So let me guess, you flunked both physics and law?
     
  11. NeverMore1701

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    Agree 100% on the 50/200 zero.
     
  12. plasticslap

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    Probably not, but I'm guessing you did
     
  13. Bushflyr

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    At no point in the previous discussion was that stated. In fact the only time you would have the barrel at 0 degrees would be if you were shooting downhill. If you are shooting level the barrel is elevated. Therefore the bullet leaves the barrel in an upward trajectory. Upward trajectory = increasing altitude with respect to the datum = rise. :tongueout:
     
  14. MisterPX

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    With 62 gr out of a 14.5 barrel, the 50M zero will have you hitting 300M keeping your dot on target.
     
  15. plasticslap

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    You just proved my point. Both of them.

    1. If the bore were at 0 degrees, the bullet would go down, as IT ALWAYS DOES. And I know it wasn't stated, it was to provide reference so that the picture would make sense. The bullet is not rising, it is in fact falling. The fact that you change the inclination of its trajectory initially does not magically give it the ability to oppose gravity and rise. You are simply exploiting what is already known as Newtonian physics of projectile motion.

    2. You did fail physics.
    :tongueout:
     
  16. cowboy1964

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    If you have to use 25 yards to zero for 50/200, how low should the POI be?
     
  17. faawrenchbndr

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    I guess your laws are wrong! :dunno:
     
  18. MisterPX

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    1/2 hte distance from your sight plane, to tehmuzzle plane. ON a typical AR, this is about 1.25 inches.
     
  19. MisterPX

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    OK knuckleheads, if your sight plane on a zeroed rifle is level, your bore is pointing UPWARDS. IN order to have your bore level, your sight plane would be aiming DOWNWARDS. YOu guys are dickin around back and forth in somebnody elses thread trying to convince each other the SAME THING.

    ETA: dammit, though bush's quote would be requoted too.
     
    #19 MisterPX, Dec 20, 2010
    Last edited by a moderator: Dec 20, 2010
  20. MrOldLude

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    This is what happens when someone who has taken highschool physics and tries to apply the most fundamental and generalized case with bad results.