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AR shoots 55-gr low and left, why?

Discussion in 'Rifle Country' started by IMtheNRA, Jan 1, 2013.

  1. IMtheNRA

    IMtheNRA Well-Known Member

    I've been scratching my head over this for a few years, so if you have a theory, I'd love to hear it.

    1994 Colt Match Target; stock Colt 20" barrel, National Match - style freefloat tube/handguards; 1/7 twist; upgraded A2-style sights with 1/4-1/4 clicks. Shooting off sandbags, zeroed in at my club's indoor 100-yard range.

    This rifle is sighted in with my favourite handloads using 69-gr SMKs and 68-gr Hornady BTHPs, both of these loads print in exactly the same location. Velocity is about 2,750 fps.

    However, when shooting any 55-gr FMJs, whether it is my reloads with Hornady FMJBT-WC bulk bullets, or surplus Lake City or IMI ammo, the groups print 3.5 inches left and 3.5 inches low at 100 yards.

    I suppose another way to look at this is to say that the 69-gr bullets print high and right...

    This difference in POI has been observed at ranges of 100, 200, 300, and 550 yards.

    What's causing such a dramatic shift in POI? :scrutiny:
    Last edited: Jan 1, 2013
  2. chris in va

    chris in va Well-Known Member

    What is the twist rate?
  3. IMtheNRA

    IMtheNRA Well-Known Member

    Twist rate of 1/7
  4. browningguy

    browningguy Well-Known Member

    The simple answer is that is where the barrel is pointing when the bullet exits. Barrels whip, a lot, and depending on where the barrel is in it's motion cycle controls where the bullet will go.

    The velocity difference between a light and heavy projectile can be quite large, that means the barrel can be in opposite extremes of it's motion when the different bullets exit.
  5. IMtheNRA

    IMtheNRA Well-Known Member

    The barrel can whip THAT much?! 3.5 inches at 100 yards...
  6. rcmodel

    rcmodel Member in memoriam

    The gun also recoils imperceptibly more with the heavier slower bullets then the lighter faster bullets while the bullet is still in the barrel.

    Were you to zero with the 55 grain bullet, it would be evident the heavier recoil of the heavier slower bullet moves the gun more before bullet exit.

    Because the groups would be 3.5 inches right and 3.5 inches higher then the 55 grain zero.

    And yes, different barrel harmonics also plays a part anytime you change loads.

  7. IMtheNRA

    IMtheNRA Well-Known Member

    So, theoretically, could I find a 55-gr load that matches the POI of my 69-gr loads? If so, what's a good place to start this load development?
  8. gdcpony

    gdcpony Well-Known Member

    I also vote harmonics.

    I once had a .243 that threw two bullets extremely well, like .5 MOA well. One was a 70gr and the other was a 55gr. But the 55gr pills were 8" high and 6" left at 200yds. The 70gr load was moving at 3500fps and the 55gr load was cooking at 4200. Obviously, there were some major differences in what was happening as those two went through the barrel. Most likely as stated your heavier bullets are leaving the muzzle at one extreme, while your lighter ones are at the other.
  9. JohnKSa

    JohnKSa Well-Known Member

    I suspect that this is more of an issue than harmonics. I think that harmonics would be more likely to induce an elevation error than a windage error.

    Try shooting the gun off the other shoulder and see if the windage error changes significantly. If it does, to the extent that it changes, recoil is the factor involved.
  10. jim243

    jim243 Well-Known Member

    Yes, a 3 MOA change at 100 yards is a 3/100 of an inch at muzzle.

    In theory, if you slow your 55 grain bullets down to the speed of your 68 grain bullets, they should "in theory" exit the barrel at the same harmonics and have the same POI. Since there are other factors involved like friction of the larger bullets and the additional pressure to push the heavier bullet, it won't be exact but should be close.

    It is worth a try using less powder for the 55 grain loads. If you have a crony then it might be easier.

    Last edited: Jan 2, 2013
  11. nipprdog

    nipprdog Well-Known Member

  12. gobysky

    gobysky Active Member

    Exactly why I like bull barrels.
  13. Boostedtwo

    Boostedtwo Well-Known Member

    I like bull barrels as well, I never knew the barrels would move like that, good video.
  14. Carl N. Brown

    Carl N. Brown Well-Known Member

    100 yards = 3600 inches.

    Typical AR barrel lengths 16" and 20"

    If my math is correct, difference in POI of 1" would be "whip" or vibration at the muzzle of 0.004444" with a 16" barrel and 0.005555" with a 20" barrel.

    That's not much.

    My son's AR carbine build was a 16" (breech to muzzle) bull barrel as free floated as possible.
  15. IMtheNRA

    IMtheNRA Well-Known Member

    Amazing video, it looks like he is shooting a rubber gun!
  16. raa-7

    raa-7 Well-Known Member

    Sure does, and it makes you think about the bbls on yer own rifles and how they react etc.Theres alot of energy moving through it for sure.
  17. helotaxi

    helotaxi Well-Known Member

    Barrel whip can occur in both planes. I've got rifles where I switch loads and almost all the difference in POI is lateral just like I've got them that are purely vertical. Most have some of both.
  18. Vern Humphrey

    Vern Humphrey Well-Known Member

    The barrel vibrates.

    Imagine a simple sine wave. Now suppose one load allows the bullet to exit at the bottom of the node, the other at the top. There will be a marked difference in point of impact.

    Imagine the nodes are .02 inches apart. The ratio of barrel length (20 inches) to range (100 yards, or 3600 inches) is 1:180. That would put the points of impact of the two loads some 3.6 inches apart.
  19. adelbridge

    adelbridge Well-Known Member

    There is also recoil spring and upper receiver flex in the equation. Piston driven uppers add even more harmonics. I love My AR's but it is some serious work to get them to shoot like a $300 bolt gun.
  20. Ar180shooter

    Ar180shooter Well-Known Member

    Just adjust your sights when going out to shoot the 55gr ammo. Since it sounds like it's a consistent 3.5", it shouldn't be a problem to reset your zero for your handloads.

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