how fast is to fast-Berger VLDs

Discussion in 'Long Range Hunting & Shooting' started by oldmossy, Mar 31, 2012.

  1. oldmossy

    oldmossy Well-Known Member

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    I'm shooting 180 grain vlds in my 7mm rum at 3300 fps. I can't get it to shoot any better than 2" at 200 yds. I have varied my loads by as much as 4 grains, and have come as far as .060 off the lands.I was told that for the barrel twist that I have, I need to be shooting the 168 vlds.
    I've heard that the vlds started coming apart anywhere from 3150-3400 fps. Just wondering if anyone could shed any light on this for me.
    And while we are on the subject of berger vlds. At what velocity does the bergers start to stay together when impacting an animal. I'm worried about shooting an animal at any close distances. I'm hunting an area where my "normal" close shot is going to be around 600yds. But who knows whats going to happen while hunting.
     
  2. Broz

    Broz Well-Known Member

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    If you push the 180's or 168 over 3250 in a barrel twist faster that 1 in 9 you will probably start to see the problems. We run them at 3150 to 3200 in 9 twists all the time with no issues. We have taken game as close as 100 yards and less with 9 twists at 3200 and all was good. But expect a very very dead animal with a big hole at these distances. This does not bother me as I never save ribs anyway. :)

    Jeff
     

  3. load

    load Well-Known Member

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    its not really coming apart but what happens with vld designs over 3000 fps is that a secondary shock wave starts appearing at the shoulder at around 3000 to 3200 fps. the longer the nose the the sharper the shoulder the worse the shock wave. This shock wave can do several things. 1 change bc unexpectantly with varying effect for atmospherics, 2 change the trajectory of bullet flight and 3 cause the bullet to loose stability.gun)
     
  4. bigngreen

    bigngreen Well-Known Member

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    Why does slowing the twist or just adding a few thou to the jacket ,with no exterior design change, help if this is what was happening?
     
    Last edited: Mar 31, 2012
  5. oldmossy

    oldmossy Well-Known Member

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    thanks for the info
     
  6. meatyrem

    meatyrem Well-Known Member

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    You never did mention what twist your barrel has?
     
  7. meatyrem

    meatyrem Well-Known Member

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    The Berger site lists the 180gr to have a 9 twist.
    And the 168gr to have a 10 twist
     
  8. rscott5028

    rscott5028 Well-Known Member

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    Not to nit-pick... but, thought it might be worth clarifying...

    Unless we're talking about rifled shotgun slugs, bullets don't usually "have" a twist.

    So, I think you meant to say that Berger recommends that you use them in a rifle with a twist that is at least as fast as those listed in order to effectively stabilize these bullets under most atmospheric conditions.

    -- richard
     
  9. meatyrem

    meatyrem Well-Known Member

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    Yes that is what was meant by that was the twist of the barrel. I'm not too sure if Berger makes slugs tho. Lol
     
  10. oldmossy

    oldmossy Well-Known Member

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    I have a factory sendero II. I think it's 1 in 9.5.
     
    Last edited: Apr 4, 2012
  11. oldmossy

    oldmossy Well-Known Member

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    Just wondering why I would be getting the speeds I am getting with my factory 7mm rum. Shooting at 2500 ft elevation. Shooting 93 grains of retumbo. Shooting 180 bergers.Getting 3300 fps. I see this is faster than all the oth er rums. Is my chrono not accurate?
     
  12. load

    load Well-Known Member

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    I dont have all the answers to exterior ballistics but one of the reasons for a slower twist and increased accuracy could be
    1- magnus effect. the faster a bullet leaves the barrel the higher the rpm it is spinning at. at 3200 fps and 1/9.5 a bullet is spinning at 242496 rpm, at 3200 fps and 1/11 is spinning at 209280. think of a top spinning it fights gravity and wants to "stand up" The more the bullet stands the harder it is to predict the shockwave/pressure areas and the more they will change with atmospherics. the slower the spin the more the bullet will want to lie parallel to the direction it is traveling. This helps with wind effects also.
    a bullet coming apart would probably start in the chamber under high pressures and may not directly correlate to velocities but may also relate to twist rate as faster rates cause higher pressures. a thicker jacket may help with this.gun)