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AR shoots 55-gr low and left, why?

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

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  1. IMtheNRA

    IMtheNRA Member

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    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 Member

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    What is the twist rate?
     
  3. IMtheNRA

    IMtheNRA Member

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    Twist rate of 1/7
     
  4. browningguy

    browningguy Member

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    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 Member

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

    rcmodel Member in memoriam

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    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.

    rc
     
  7. IMtheNRA

    IMtheNRA Member

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    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 Member

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    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 Member

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    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 Member

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    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.

    Jim
     
    Last edited: Jan 2, 2013
  11. nipprdog

    nipprdog Member

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  12. gobysky

    gobysky Member

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    Exactly why I like bull barrels.
     
  13. Boostedtwo

    Boostedtwo Member

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

    Carl N. Brown Member

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    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 Member

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

    raa-7 Member

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    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 Member

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    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 Member

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    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 Member

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    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 Member

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    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.
     
  21. MarkA

    MarkA Member

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    ^^This.. Every barrel has whip. Every barrel is going to behave differently, even on identical firearms. This is why ladder testing or round robin testing is so effective in load development. There's nothing you can do to eliminate the oscillation in the barrel, but minute changes in powder charge, powder type, primer type, etc.. can affect when the bullet exits the barrel relative to the position of the end of the barrel in it's oscillation cycle. a 3200 fps projectile will make it down a 20" barrel in 520 microseconds. The oscillation period is some other minuscule measure of time... so theoretically you want your projectile to exit the muzzle when the barrel is in it's neutral position.

    Thinking back to some of my latest .308 adventures, walking powder charges up in .5 gr increments for a "rough in" of where I wanted to tweak more accurately, I was producing groups that would start small, gradually get larger, then shrink down again but on average be higher on the paper. It proves the theory true for the most part. I was walking back and forth over that 'neutral point' while increasing my average velocities thus hitting higher and higher on the paper as I went. This isn't as easy to see at 100 yards, which is why I do all my load testing at 200 or 300 yards depending on the weather (wind mostly).
     
  22. Ramone

    Ramone Member

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  23. IMtheNRA

    IMtheNRA Member

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    It is interesting to see the second target in that link, which shows a low+left group of M193 ammo from a 1:7 twist M16A2, which is the same twist rate and similar barrel length to my test rifle.

    The article discussed only optimal rifling twist rates, and made no mention of barrel harmonics.

    Frankly, I am not yet ready to blame the group shift only on barrel harmonics because I observed the same 100-yard POI with THREE different handloads of the Hornady 55-gr FMJBT-WC bullet - two different charges of TAC and one charge of Viht-133.

    Statistically speaking, isn't it more likely that the three loads would cause the bullet to leave the barrel at three different points in time rather than during the same point in barrel oscillation?
     
  24. helotaxi

    helotaxi Member

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    Case in point, I was doing load development in my 6x45 rifle yesterday and normally what you expect is the groups to move up and away from the shooter (left in my case as a lefty) as the charge weight increases until it hits the top of the barrel node and then start coming back down. This rifle shifted straight left almost 3" over a 1.5gn increase in powder weight.
     
  25. chrome_austex

    chrome_austex Member

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    I've seen plenty of left-right POI shifts between ammo types in my own testing at 200y.
     
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