not accurate at 100 but accurate at 200

Discussion in 'Long Range Hunting & Shooting' started by startrek1761d, May 15, 2014.

  1. startrek1761d

    startrek1761d Well-Known Member

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    Someone please explain to me how when a bullet leaves the influence of the barrel and is headed down range and becomes stabilized the groups can be smaller at longer ranges, the bullets hitting the 100 yard target don't know to correct their trajectory and get back on target at 200 yards. I know it takes some bullets longer to stabilize but once the path of the bullet is set why wouldn't the 200 yd group be double the size of the 100 yd group,after all the bullets don't know where the target is at 200 yards and couldn't correct their trajectory if they did.
     
  2. dragman

    dragman Well-Known Member

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    They don't.
    Honestly sometimes bullets stabilize slowly and maybe only a 1" gun at 100 but maintain that out to 3-4 hundred. So it doesn't get worse but it doesn't ever get better.
    Parallax, mirage, different angle something else is causing what your seeing
     
  3. gilmillan1

    gilmillan1 Well-Known Member

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    I have experienced the same thing. I have a 243 that does .8-.9 MOA at 100 yards, but shoots .6 MOA or better at 600 yards. I dont know what it is, but I feel very confident shooting it at long range. Just enjoy your rifle and dont be too concerned about groups or trying to figure out why it shoots better at long range. in the end, thats what we want: a better group at long distances. Dont break your head trying to figure it out, just accept it.
     
  4. WildRose

    WildRose Well-Known Member

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    If a guy is shooting heavy for caliber bullets or VLD's or some other very long bullet with the wrong twist or a twist rate that is right on the verge of enough for that velocity it's possible.

    In general though I think you are right, parallax is the most likely cause if it's not just shooter error.

    It's not the least bit uncommon for people to kind of fling it at close range which produces open groups and then focus and hunker down and really try to get things right at longer range either.
     
  5. Bullet bumper

    Bullet bumper Well-Known Member

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    It could also be " In bore yaw " no that's not a foreign language or a southern accent . It means when a bullet does not go down the bore perfectly in centre so when it exits it can wobble or gyrate to some degree , but as the range increases the spin rate tends to settle it down .
    Dynamic Stability . The ability of a projectile to dampen its initial wobble and thereafter fly straight and true to the target.
    Check your twist rate is good enough for the bullet length , check your load is hot enough to get the bullet expanded to the bore , check your barrel is not over fouled with copper . Check that the bullet is concentrically seated . Try moving it closer to the lands . If the barrel is worn then go to a flat based bullet .
     
  6. davkrat

    davkrat Well-Known Member

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    I know most folks say its impossible but my .308 shoots the exact same size groups at 400 as it does at 200. Maybe its parallax. Could it just be me? Could I be that consistantly inconsistent?
     
  7. startrek1761d

    startrek1761d Well-Known Member

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    Here's what Berger said, I don't get it. It has been reported that the VLD bullets don’t group as well at 100 yards but get better as the bullet “goes to sleep” at further ranges.
     
  8. Bullet bumper

    Bullet bumper Well-Known Member

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    Bullets with very short bearing surfaces and Secant ogives are more prone to , in bore yaw , the wobble produced tends to dampen out some what down range with Dynamic stability taking over . Not good bullets for rough handloading , worm or oversize bores , low pressures , jumping to the lands , short ranges , bad crowns , badly fouled bores , incorrect twist rates .
     
  9. startrek1761d

    startrek1761d Well-Known Member

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    But how does the bullet become more accurate at longer ranges, it cannot correct it's path and steer towards the target.
     
  10. Bullet bumper

    Bullet bumper Well-Known Member

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    Simple , the bullet starts out the barrel with a wobble that will cause group expansion at close range but as the bullet progresses down range the Dynamic stability gets over the wobble and settles the bullet down so groups tighten suddenly .
    The factors listed above can make it worse or better depending on which way they are .
    Many people are using a flat base bullet with no real problems but then they try to use a VLD the same way and some times the launch conditions are not suitable for that design to work it's best .
    Don't get group accuracy and basic POI Zero accuracy mixed up. Slightly different things. You can shoot a tight group but still be feet away from where you should be zeroed . You can be zeroed as best you can but still fire a 3 foot diameter group all around that zero . Except for wind influences and spin drift the bullet can not change it basic line of zeroed trajectory outside of it's group spread potential . So if you are zeroed 6 inch to the left of the target centre then you are going to group to the left if no wind or spin drift does otherwise but the overall group size could change .
    That is the good thing about being able to shoot a tight group you can then adjust a more precise zero , not withstanding click value .
     
  11. FearNoWind

    FearNoWind Well-Known Member

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    I believe Berger is describing the Dynamic Stability issue that Bullet Bumper mentioned above to explain that the bullet develops a slight yaw which is eventually overcome and the "excitement" in the bullet's dynamics settles down to stabilize it's flight. (read physics descriptions on gyroscopic precession and angular momentum).
    If you spin a top or gyroscope it may initially wobble but it will often stabilize itself without outside influence.
    The initial instability is relatively insignificant for the hunter who is unlikely to need 1/4 MOA at 100 yards in a hunting situation but will certainly need a higher degree of accuracy at longer distances.
     
  12. mrb1982

    mrb1982 Well-Known Member

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    I read this thread last night. I was kinda under the impression that it was a bunch of BS as far as the whole thing about going to sleep and all that.

    So this owning I tool my 7RM Sendero out. I have been working up a load with 180 Berger Hyrids. Did the seating depth test, then worked up my charge and ended at 70gr H1000. The best I could do with this load at 100 yds was 5/8 MOA.

    So I start by setting my 200 yard zero. 3 bullets touching about 1" high and 5/8" to the right. Best accuracy I had out of all the test loads. Then I was starting to believe.

    So I back it up to 400. It shot .43 MOA from there. Pretty pleased to this point. I don't have a chronograph so I have to reign in my MV the old fashioned way. This has given me a fair velocity to get going back to 700. That should give me a better number but this is a smoking hot load. No ejector marks or other brass problems. Just starting to get a hair of a primer flattening. We'll see how it runs out to 700.

    I am now a believer in the "yaw" theory by to some extent. I'd think there are probably some other factors that come in here too as far as the mental side of shooting and whatnot but I now believe it has viability
     
  13. Punisher

    Punisher Well-Known Member

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    +1

    I'm betting that you are seeing a problem with in bore yaw, or twist rate/ velocity combination. You can try the following:

    Decrease the velocity of your load because your twist isnt fast enough to stabilize flight at that speed.

    Change your bullet. Flat based bullets really are responsible for the best 100 yard groups according to my benchrest shooting neighbor, and I really trust the old guy. He has the trophies to back it up.
     
  14. stephenthesuave

    stephenthesuave Member

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    Alot of people smarter than me think the bullet can go to sleep and be more accurate at longer ranges than shorter ranges. One distinction that some folks miss is that being more accurate does not mean smaller groups. In the example a few posts back, the guy was shooting 5/8 MOA at 100 but .43 MOA at 400. It's not like the bullets corrected their course, the group at 400 is still bigger than the group at 100.

    I've never seen someone claim a 1 inch gun at 100 was a half in gun at 200. That would take some kind of outside force that I don't believe exists.