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Thread: Question regarding twist, (rifle vs. handgun)

  1. #1
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    Default Question regarding twist, (rifle vs. handgun)

    OK, so the required twist for a particular bullet, is based on itís length, and to a lesser degree on itís velocity.

    BUT, the velocity is LESS in a handgun barrel.

    Also, there may be less opportunity for the twist to take effect in a handgun barrel.

    So, with these rifles that shoot handgun cartridges, (examples, 38-40, 44-40, 357, 44 Mag., 45 Colt, etc.)

    Isssss,,,, it a good idea to use the SAME twist, that is used in a handgun? (Assuming the same bullets, that are used in a handgun)

    When using heavier bullets than would be used in a handgun?

    If a handgun twist is adequate for a particular bullet, would it be adequate for a rifle, also, or vise versa?

    I don't wanna talk about what prompted this question, because I wanna narrow focus, on one thing at a time.

    If you have something to offer, regarding any possible issues, with this, I'd appreciate hearing it.

    Thank you.

    Smitty of the North
    Walk Slow, and Drink a Lotta Water.
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    Sponsor ADfields's Avatar
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    Good question Smitty. I donít even know the rate in my handgun ammo shooting lever guns and their lack of superior tackdriverness could be due to twist. They arenít bad just not great, maybe 3 MOA but with open sights and slow calibers thatís not bad for what I use them for. Iíll be checking their rates and considering now.
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    Each bullet has a specific rpm at which it will stabilize. So long as you meet or exceed the rpm it takes to stabilize a bullet it will be stable. You can increase rpm by either increasing bullet speed or by shooting in a gun with faster twist. Normally rifles give more velocity so may stabilize a bullet that is marginal in a handguns shorter barrel. When talking handgun cartridges such as your examples the handgun twist should do fine in a rifle as the velocity and rpm should be higher. When shooting heavier bullets in a rifle you may need to go to a faster twist if the velocity increase is not enough or you have lost velocity because of the heavier bullet, to hit minimum rpm. Twists that stabilize a bullet at rifle velocity may not stabilize it at the lower speeds of a pistol barrel. It is possible to have to much twist or velocity and cause the cast bullet to skid in the first inch or two of a barrel causing leading but that is normally in rifles at relatively high velocities (2000+) and twists of 1/10 or faster. Any barrel length or twist should impart the full amount of twist on the bullet unless it skids completely out the end of the barrel. That's very unlikely. Some times I don't explain things very well so If I have confused you worse than ever I apoligize.

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    rbuck351:
    You explain things well.
    Thanks
    Smitty of the North
    Walk Slow, and Drink a Lotta Water.
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    Smitty,

    Your question interested me to the point that I've spent the last two nights researching it during down time at work.

    I've recently finished my second reading of a book by Bryan Litz named "Applied Ballistics for Long Range Shooting".

    http://www.appliedballisticsllc.com/...files/Book.htm

    Your question is not directly covered in the chapters of this book but the answers lie in between the subjects covered in chapter 10.

    Brian is an Aerospace Engineer that worked in missile design for the US Air force. He is a champion 1000 yard Palma match shooter and the current head ballistician for Berger bullets.

    You are correct that length not weight determines twist rate requirements. However, length is generally longer as weight goes up unless you are comparing against a 100% copper bullet versus a lead core design.

    A barrel as short as 1.5" can adequately stabilize a bullet if the bullet's length is proper for the twist rate. More velocity does not always increase stability in a barrel that has the wrong twist rate for the bullet selection. In one example in his book a 155gr bullet was increased in velocity by 10%. The increase in velocity actually increased the drag that tried to upset the bullets nose forward flight position.

    A bullet shot with excess velocity will fly with some amount of pitch and yaw until gyroscopic stability improves enough thru loss of velocity to restore point forward flight. This is sometimes referred to as "going to sleep". In cases where twist rate is increased and the bullet is "more stable" it doesn't acquire more drag but it is more resistant to tumbling.

    In your environment you are subjected to more air density issues, thicker air. You are nearer sea level than most and shooting in colder temperatures than most. Dense air causes more frontal drag on bullets and they are more likely to tumble if a less than adequate twist rate is selected for your length of bullet. That same twist rate might preform just fine at 5000 feet in thin warm lower 48 air with less drag trying to upset your bullet path.

    A poorly designed bullet that is unbalanced about it's center of mass can cause stability issues too, much like a tire and wheel spinning on a car with out a proper balance. Those bullets can shoot better out of a slower twist barrel that does not aggravate the imperfections much like that car scenario doesn't shake you around while driving at low speed.

    Since you mentioned a rifle shooting pistol bullets I will assume that you are shooting a short fat bullet not a long pointed bullet. In that scenario twist rate is rarely a factor since bullet design is centered around caliber and typical twist rates for that application are general well thought out. The longer barrel on the rifle gives you more options for increased velocity with improved powder selections.

    The old Greenhill formula has been improved for predicting stability. The Miller Stability Formula more accurately predicts twist rate requirements for optimal bullet stabilization.

    I'm not sure what bullet you are using but keep in mind that Barnes bullets are typically 18% lighter than equivalent length lead core bullets. If you are using one of the heavy Barnes bullets you may be to long for your twist rate.

    If you provide me your caliber and length in inches down to 0.000" I will dust off my old Texas Instruments solar scientific calculator and plug in the formula to calculate the optimum twist for your question.

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    I have read that some rifles shooting pistol calibers have very slow twists and although they shoot the std weight for caliber bullets well, they don't shoot the heavy (long) ones well. My memory isn't the best but I believe there are some 44mag carbines that have twist rates around 1/28 which shoot the 240gr bullets ok but don't do so well with the longer 300+gr bullets. There are probably other examples that I'm not aware of as well.

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    Art I think your response to my response is more directed towards long range pointy boat tail rifle bullets.

    The increase in rotational velocity compared to forward velocity tends to increase the nose up position. The more the nose rises the greater the drag and the faster forward velocity slows. That is the only function where excess spin works against accuracy or bullet speed.

    Based on his published writings I would have to disagree with your response. It is drag from increased velocity that causes the nose to tilt up not increased spin. Once the bullet settles down or "goes to sleep" drag is reduced as the bullet is flying true.

    You have to be running on the ragged edge of instability before barometers would show much difference in reasonable bullets.

    I agree but we still don't know what bullets Smitty is talking about. I'm throwing it out there as an example. If he was just barely stable at 5000msl in warm air there is a chance he would not be stable in cold sea level air.

    Your follow-up point about being "more stable" is largely incorrect because the drag obviously increases... and as a function of the square of the velocity. But the point is true that the bullet may stablize if shot faster... again disproving the point about the bullet shedding excess velocity to stablize.

    Stabilize if shot faster? A 10% increase in velocity only offers a 3% increase in stability.

    What happens when it slows? If the extra velocity got it to stabilize it will tumble when it slows.

    The author goes into the pros and cons on both sides of the coin. He largely disproves your position by graphing a negligible increase in drag caused by an increase in spin versus a huge increase in drag caused by an increase in velocity. He claims the increase in drag is what causes the bullet to fly nose high as the center of lift moves back on the long bullet. As velocity drops and the spin continues it becomes stable and flys true. Don't confuse this with velocity dropping to a point less than super sonic as trans sonic flight causes other issues.

    Because the bullet is advancing through the air there is a greater pressure on the front, bottom side of the nose, and a lower pressure behind the bullet on the top side. It is that force which holds the bullet slightly nose-up. The farther downrange the bullet goes the greater the force is because rotational velocity has virtually no resistance while velocity is shed fairly rapidly.

    I agree with the pressure points and locations but the spin has very little resistance and the bullet nose will drop as drag is reduced by less velocity down range.

    It's my guess at this point that Smitty is shooting short, fat, blunt nose bullets and very little of our conversation really applies to his question. After all, if he is shooting short, fat, heavy bullets they are inherently easy to stabilize.

    In closing I believe Brian's book is a good read and offers a lot of information and scientific calculations to prove and disprove a lot of ballistic data. His 1000 yards championships prove his ability and his projectile design expertise proves his knowledge.

    I will bow out and enjoy the rest of the book...



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    Marshall:
    Thanks, that is the kind of detailed information I need. Iíll try and relate it to my situation.

    I think that I recall reading that the twist in a ML pistol barrel needed a different twist than a rifle. (Of course, this could be due to the very slow twists needed for Round Balls, and doesnít apply to modern handguns versus rifles.)

    hap:
    and
    Marshall:

    Well, I didnít identify the bullet, or other specifics, because, I donít know the length of the bullet, and Iím not certain of the twist, so not wanting to ask without the correct information, until I know those things, I thought Iíd just learn more about issues with twist, in long vs. short barrels.

    My bullets are all loaded, and Iíd hafta pull one to measure it, or wait until I can get some more. It is the Cast Performance, 357, 200 grain WLNFP GC cast bullet.

    I dunno, what to think. I loaded them for my Ruger Blackhawk, and they seemed to be accurate enough at close range.

    When I fire them in the rifle, with a 20 barrel they arenít accurate, and the bullets keyhole.

    Iím think twist could be the issue, but with cast bullets, I thought it was possible, there could be other issues.

    Smitty of the North
    Walk Slow, and Drink a Lotta Water.
    Has it ever occurred to you, that Nothing ever occurs to God? Adrien Rodgers.
    You can't out-give God.

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    Since discovering the manufacturer of my rifle says the twist is 1-30, I'm satisfied the twist is insufficient for the long 200 grain 357 bullets.

    Because of what I read somewhere long ago, about ML pistols, I wondered if there was a difference in twist requirements, rifle versus pistol. (I have no idea why a rifle would have a 1-30, and a handgun 1-16.)

    Thanks to everyone.

    Smitty of the North
    Walk Slow, and Drink a Lotta Water.
    Has it ever occurred to you, that Nothing ever occurs to God? Adrien Rodgers.
    You can't out-give God.

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    I think the problem is engineers not really realizing what we as public are planning on doing with the product they have designed. The engineer that decided on the 1/30 twist probably wasn't even aware that 357Mag came in 200gr. Unfortunatley the engineers that design things don't have to work on them and may not even use them. College education does not encourage thinking out of the box. A good grade comes from getting the right answer and by doing it the professors way. You, by shooting 200gr bullets, are thinking out of the box. I'm with you on your thought that a 200gr would be more effective for what you want to do, but the engineers narrow minded thinking (357=158gr bullets) didn't allow him to think out of his box. Manufactures should ask us what we want instead of building what they think we should want. A 1/30 twist just seems dumb to me when a 1/16-1/18 would work better over a wider range of bullets.

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