velocity in relation to barrle twist

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Crazycooydog, Feb 3, 2004.

  1. Crazycooydog

    Crazycooydog Active Member

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    Dosen't velocity determine twist rate needed to stablize said bullet.

    EG: 223 with 1-12 twist 55gr bullet 3200 fps
    223 with 1-12 twist will not stablize 70gr bullet(no clue as to how fast it would go)
    22-243 with 1-12 twist 55gr bullet 4100 fps

    22-243 with 1-12 will stablize 70 gr bullet at 3800.(maybe not to the point of .5moa acuracy)
    This is a poor eg. but i hope ya get the idea.
    Is'nt it RPM's that stablize the bullet.If so less twist at higher velocity is needed to stablize the same bullet.

    Is'nt this true?????
    [ 02-03-2004: Message edited by: Crazycooydog ]

    [ 02-03-2004: Message edited by: Crazycooydog ]
     
  2. Varmint Hunter

    Varmint Hunter Well-Known Member

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    Twist rate has FAR more influence on bullet rotation rate than velocity does.
    Most barrel manufacturers only make recommendations for twist based on the bullet weight/length and disregard potential velocity.

    Many 22 centerfire rfles have a 14" twist regardless of whether it is a 221 or 220Swift.
     

  3. Mikecr

    Mikecr Well-Known Member

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    Varmint hunter
    Isn't gyroscopic stability proportional to RPMs?
     
  4. Varmint Hunter

    Varmint Hunter Well-Known Member

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    Heavier/longer bullets do, indeed, require a higher rotational spin on the bullet to maintain stability.
    My point is that a velocity change +/- has a much smaller influence on the rate of that spin than does a minor change in twist rate.
    I shoot a 22-243AI and 80gr VLD bullets. This rifle needs an 8" twist to stabilize these heavy/long bullets just the same as a .223 would. Even though the 22-243AI can shoot them at 3,600+ ft/sec, which is MUCH faster than a .223, I still need the same 8" twist. Would the extra velocity allow me to use a 10" or 12" twist barrel - No. I could probably get by with a SLIGHTLY slower twist but this would be insignificant.

    VH
     
  5. Crazycooydog

    Crazycooydog Active Member

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    Yes I understand that certain twist's are needed for certain bullets. I just was wondering how a 223 shooting a 70gr bullet at a MUCH slower velocity, than say a 22-243 with the same bullet at a MUCH higher velocity would use the same twist barrle.

    I'm talking a velocity difference of like 6-800 fps. It's got to make a difference.

    Just thinking of my factory rem 22-250 1-14 shoots 50gr v-max's awsome,it all so shoots 55gr very good.
    1-14 twist is recamended for 50-52gr bullets...right?(Lilja's recamendations)
    1-12 for 55-60gr bullets
    1-9or10 for 68-69gr bullets

    Varmint Hunter
    For sure in a higher velocity 1-9 twist. ???Couldn't ya?????
    I am just trying to understand this whole concept. Looks like it's the rpm's that are what stablize the bullets.Larger bullets need more rpms to get them to stablize.
    There fore is there such thing as over stablizing a bullet.I think yes because it comes apart if it spins too many rpm's.. ????Does'nt it????

    [ 02-03-2004: Message edited by: Crazycooydog ]

    [ 02-03-2004: Message edited by: Crazycooydog ]
     
  6. Varmint Hunter

    Varmint Hunter Well-Known Member

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    Try not to take what I have stated out of context - A change in velocity does change the rotational speed of the bullet but just not enough to reqire a different twist rate. At least not in a reasonable comparison.

    Yes, if you take it to the extreme, like shooting the same bullet in a 218 Bee and a 220 Swift you MAY be able to use different twists. However, they would likely both work quite well with the same twist.

    To answer your other question: An 80gr VLD will not stabilize in a 10" twist, not even in the 22-243AI. The 9" twist is something that I'd have to try, but I doubt it.
    Now how about a 8.5"? [​IMG]

    Here is something else to think about: When shooting over looong range there is not only velocity loss but rotational spin decay. Do you need to increase the rotational spin at the onset in an effort to compensate for the inevitable decay? [​IMG]

    VH
     
  7. Crazycooydog

    Crazycooydog Active Member

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    I realy know nothing about this whole twist thing.

    Here is the delema:
    I am building a 22-243
    I would like to shoot 55gr gamekings FOR SURE
    May like to try 68's for wind but realy don't want to take anything away from the 55's.

    [​IMG] What to do [​IMG]
     
  8. Michael Eichele

    Michael Eichele Well-Known Member

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    For optimum accuracy, BC, stability, and overall performance a twist is determind by bullet configuration, its specifec gravity and its velocity. If you have to choose only 1 parameter, then you would choose a twist that will stabilize your bullet based on its length at any velocity. If you have a choice, its better to calculate everything. If you really want to get techincal, you would even figure in the typical air desity you shoot in.

    Lilja has a free download that will help determine your twist. Also on the jbm ballistics page, there is a calculator for twists. That is probably the best one. Also the RSI shooting lab has a twist calculator as does the Reloader's Archive.
     
  9. Varmint Hunter

    Varmint Hunter Well-Known Member

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    I would put the most faith in the opinion of your smith. Presumably, he has built similar rifles before.
    Mike Bryant (Bryant Custom) has built many 22-243 rifles. I'd guess that the majority of them have been fast twist barrels for the heavy bullets but some guys just love those fast a$$, screaming-out-the bore light bullet rigs. [​IMG]

    If I were you I would be looking at a 9" twist. This should shoot everything between 50gr - 75gr. This big cartridge really shows its stuff with the heaviest bullets. If you get a twist that is only fast enough for the light bullets, you will never be able to shoot the heavys.
    Just my 2 cents.
    Good luck with your project rifle.

    VH
     
  10. Michael Eichele

    Michael Eichele Well-Known Member

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    Just a quick example of twist, velocity and RPM:

    30 cal 175 at 1.250" long. 11 twist at 2588 FPS= 169396 RPM

    The same bullet in a 13 twist at 3058 FPS= 169366 RPM

    Thats 2 inches of twist differance for just under 500 FPS.

    I also agree with VH, if you have a trust worthy smith, talk to him.
     
  11. Crazycooydog

    Crazycooydog Active Member

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    meichele
    Thats what I was thinking. will them two barrles shoot the same????? [​IMG]

    Thanks VH

    I am not questioning you just trying to understand.
    This talk has been very helpfull.
    Maybe 1-9 is the answer for what I would like.
     
  12. Michael Eichele

    Michael Eichele Well-Known Member

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    First ask your self what bullet you want to use. Then ask yourself, how fast will a given rifle push that bullet saftley. Then you can come up with a good twist. If you get me a bullet length, diameter and its velocity, I can do a calc. When in doubt, go a little faster.

    I need bullet length, nose length, boat tail length, end diameter, meplat diameter ogive radius and the specifec gravity (what is the bullet made of? solid copper, jacketed lead, hollow point jacketed lead, nylon tip? ect...) and the velocity you expect to get, realisticly. Also 2nd any opinions you may get from me.
     
  13. Mikecr

    Mikecr Well-Known Member

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    So is stability based on;

    Revolutions per TIME?
    or
    Revolutions per DISTANCE?
     
  14. Crazycooydog

    Crazycooydog Active Member

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    I think it is based on rpm.


    meichele

    Maybe 3500fps is more realistic.