twist rate

Discussion in 'Long Range Hunting & Shooting' started by rufous, Jan 17, 2002.

  1. rufous

    rufous Well-Known Member

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    While I have a good load worked up for my 300 Win Mag with the 220 Sierra Matchking I am considering a lighter yet high bc bullet that will give flatter trajectory. As mentioned in my previous post I can only rezero up to 550 yards and still be within the hash marks on my scope. I called Barnes and they told me that their 30 caliber 200 grain X bullet is 1.476" long and that it only needs a 1:10 twist to stabilize (presumably even at 30-’06 velocities). Warren Jensen has told me that a 1:10 is fine for his 180 grain J36 bullet. I do not know how long it is but I imagine it is more than 1.4". I have some of Gerard Schultz's 160 HV bullets on order. I would have ordered the 173 HV but it is 1.496" long and he said that I would need a 1:9 twist. I would really rather use the 173 than the 160 because even though it starts out slower it ends up surpassing the 160 due to its much higher bc (650 versus 515).

    For example, starting the 160 at 3400 and with a zero of 275 yards:
    At 700 yds velocity = 2180 fps; energy=1690; drop=65” and drift=27”

    The 173 at a muzzle velocity of 3270 would have the following figures at 700:
    Velocity=2300; energy=2040; drop=65”; and drift=22”

    So clearly the 173 outperforms the 160. Now for the big question- does one really need a 1:9 twist for the 173 HV, especially in light of the fact that the 220 Sierra Matchking (one guy I have talked to is shooting the 240 Sierra Matchking from a 1:10 twist 300 Win Mag without any instability), the 200 Barnes X and the 180 J36 will all stabilize just fine from a 1:10 twist?

    Any thoughts? Thanks, Rufous.
     
  2. Warren Jensen

    Warren Jensen Well-Known Member

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    Rufous,

    The length and weight of a bullet are not the determining factors in gyroscopic stability. It is the relationship between the center of reverse air pressure under the ogive and the center of mass. The actual equation is fairly complex. The greater the distance between these two the faster the twist(spin) needed to keep it pointed nose forward.

    If Gerard recommends a 1:9 for best performance then I would believe him. He's knows more about his bullets than anyone else. If you don't think he's correct, about the only way for you to find out is to purchase some and shoot them. Sometimes things don't always work out the way the math says it should. That is usually because there are factors in the equation that were not properly accounted for.

    Warren
     
  3. rufous

    rufous Well-Known Member

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    Warren, thanks for the info. As I remember it, Gerard did say that his 30 cal 173HV would probably shoot accurately enough in a 1:10 but that it may tumble when it is penetrating through the animal. Obviously it would be simple enough to shoot the 173 grain bullets and see if they group well in my rifle. The thing I am so nervous about is the question of whether or not they would tumble once connecting with the animal, thereby possibly not penetrating enough. I do not know what to think. I would love to try his 173 HV (honestly I would love to try your 180 J36 but do not think I can swing the cost sorry to say. I do not doubt that they are worth the cost but I am on a tight budget). Clearly I want to put the animal down as quickly as possible. There are so many options and very few (if any) are perfect. It seems no matter what choice is made there is a compromise. I am not an ultralong range hunter (I think 500 yards is a very long shot on big game under the limitations I have of hunting on foot with a 9.5# rifle). The scope I plan to use does not have a target turret that can be used to rezero at ranges beyond 550 yards for the 220 Sierra or 650 yards for a bullet such as the 160 or 173 HV or your J36 bullets. So maybe it would be best to just shoot the 220 Sierra and figure if the animal is beyond 550 I need to get closer or pass it up. Trying to figure out the way to go, Rufous.
     
  4. Warren Jensen

    Warren Jensen Well-Known Member

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    Rufous,

    You can test to see if they will tumble in media. Use wet newspaper,phone books, or catalogues. Don't stack them neat. You need at least 40" depth of material. In the first 2" of material place a 2" green pine board with a 30 degree cant angle. This will simulate a bone, but the wood has to be uncured and green. Mark the front of the material so that you will impact at the correct place and angle to stay in the media. Don't forget to allow for the scope offset above the barrel centerline.

    If it maintains a straight line through this concoction then you do not have to worry about it's performance in game. It will do, OK.

    Warren
     
  5. rufous

    rufous Well-Known Member

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    Warren, thanks for the tip. Why is it necessary to cant the board? Bones are not canted at a 30 degree angle. If I cannot easily find an uncured green pine board could I just try this in wet paper? Would the bullet be more likely to tumble going through a board (or bone in the real world example) than just through wet paper (or hide and flesh)? I have never shot a bullet into wet paper. How would I tell if the bullet had tumbled? If it is sideways or backwards when I find it then it would be obvious that it had tumbled but what other clues might I look for?

    I do not understand your last sentence. Are you saying that you are quite confident that I will find that the 173 HV will not tumble and will in fact be quite stable in a 1:10 twist? Thanks again, Rufous.
     
  6. rufous

    rufous Well-Known Member

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    Warren, another question- at what range should I do this? Obviously I would not want the bullet to tumble at any range but is it safe to assume that if it does not tumble at muzzle velocity then it will not tumble at slower velocities? Thanks, Rufous.
     
  7. Dave King

    Dave King Well-Known Member

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    "The length and weight of a bullet are not the determining factors in gyroscopic stability. It is the relationship between the center of reverse air pressure under the ogive and the center of mass. The actual equation is fairly complex. The greater the distance between these two the faster the twist(spin) needed to keep it pointed nose forward. "


    Warren, I love it when you talk like that!!

    In my mind's eye I always equated the twist rate required as a function of length as I imagined trying to spin a squatty toy top in comparison to a #2 Lead Pencil. The spin required for this is that required to overcome only gravity. I never gave too much thought to the pressure caused by the bullet flying through air. Good thing I'm not a rocket scientist!!

    rufous

    I think the board requires the cant because many bones a round(ed), not flat. It'd be more common to bit a bone off-center than directly at 90 degrees to the flight of the bullet.
     
  8. Warren Jensen

    Warren Jensen Well-Known Member

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    Sorry, for the delayed response. I have been on a business trip.

    Rufous,

    This is not meant as a personal comment so please don't take it as such. As with many intelligent, inquisitive people you tend to overthink the problem. You are trying to interpret the results without having completed the experiment. First things first. You will answer some of your own questions and I will help with the rest.

    The cant in the board is meant to introduce a destabilizing factor, as many hits on animals will. The worst result from yaw and tumbling in tissue is a deviation in path, but depending on many factors it can be quite beneficial to the overall lethality, if the path is straight. Determine that first.

    Lets try to answer the "what ifs" as "whys".
    Warren
     
  9. rufous

    rufous Well-Known Member

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    Warren, thanks for getting back to me on this. I am still hoping for some more info before starting this test. I do not have any of the 173 HV bullets ordered yet so it will be quite some time before I can do it. I thought I would see if my gun shoots the 160 HV accurately before ordering the 173 HV and who knows when the 160s will arrive? As these bullets are hard to get and fairly expensive I do not wish to shoot more than necessary into wet paper. Should I just do it at 10 feet or at 400 yards or what? Again I wonder what you meant by saying "It will do, Okay"? Please clarify. Thanks again, Rufous.
     
  10. Warren Jensen

    Warren Jensen Well-Known Member

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    Rufous,

    What I meant is what I said. No codes, no secret handshake. If when trying to make it deviate from a straight line path, it doesn't, then that is in of itself a very good indicator that it did not destabilize in the media. This in turn is a good indicator of how it will perform in game. No path deviation = good performance in this case.

    A recurring theme in your questions is the cost involved. To gain knowledge you have to spend time, effort, or money to acquire it. It is either that or believe someone who has spent the time, effort, and money when they share that information with you. If you cannot do either, then you should not start. I personally, am inherently dubious of second hand data. The more data, the more reliable the conclusions. Do not fall into the trap of drawing conclusions based upon a sample population of one or two.

    If you want to know what it does at 10 feet, shoot it at 10 feet. If you want to know what it does at 400 yds., shoot it at 400 yds.

    [ 01-23-2002: Message edited by: Warren Jensen ]
     
  11. rufous

    rufous Well-Known Member

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    Warren, I am not trying to be difficult. I am trying to learn. That is why I and probably most people frequent sites such as this. Naturally I want to learn what I can from others' experience. I do not have unlimited time or money. If those who have done such experiments find that if the bullet will tumble at 10 feet then it will tumble at all ranges it would be futile for me to do it. Yes, then I would know first hand but I do not need that in all cases. It is ideal but not always necessary for me.

    I have read many articles and posts by hundreds of people who say that Nosler Partitions are excellent bullets. I have used them myself and found the same thing. I have not read articles or posts saying that the GS Custom 173 HV 30 caliber bullet needs a 1:9 twist to prevent tublimg in game except by Gerard himself. I admit that he would be the first one to listen to, but based on other information I have on bullet lengths and required twists for them I do wonder if a 1:10 would suffice. I had hoped to hear from others who had shot game with a 300 Win Mag 1:10 twist using the 173 HV and either found good or poor performance. I would never have considered using Sierra Matchkings on game. It goes against all that I have read and learned over the years about necessary bullet construction to be used on big game. But there are so many who are using these "Match" bullets successfully that I may give them a try.

    I was not looking for some secret handshake. I am just trying (still) to understand what you were trying to say. If we were face to face I have no doubt that I would understand what you were saying. The written word sometimes does not come across as easily understood. You said "It will do, Okay?" What I am still wondering is what is the "it" that you refer to and what is it that the it will do? Is it simply that if I do this test and find that the bullet does not tumble then I should be confident in using it for hunting? I appreciate your patience and your willingness to pass on to the rest of us your findings. Rufous.
     
  12. Jon A

    Jon A Well-Known Member

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    rufous,

    I've kind of had this debate with myself already. Of course testing is always the best, but as you know GS bullets aren't cheap and they take quite some time to get....

    I think the answer depends upon what you're hunting and what shots you're willing to take. I'm pretty sure I've heard Gerard say the 173's would be just fine in flight from a 300 Win with a 1:10. The problem is tumbling after impact.

    If you're hunting deer or smaller game and limit yourself to broadside shots, then I would guess the bullet would do just fine whether it tumbles or not--dead dear!

    But, if your style of hunting could have you following the trophy of a lifetime through the brush showing you nothing but his ass, if you want to take that shot you need deep, straight penetration--you can't have the bullet tumbling.

    Personally, I'd take that shot. I want a bullet that will reliably penetrate from stem to stern--just in case. That's why I decided on the 160's. I don't have the time or money to test everything, so I figure Gerard wouldn't recommend what he does without good reason. That's all I have to go on. I know the high BC of the 173 is tempting, but when its maker recommends against its use from your rifle...is it worth the risk?

    FWIW, I think one of the biggest reasons he is so strict about twist recommendations is how long his bullets are--after expansion. Most bullets will be reduced to about 1/2 their length or even less after initial expansion. His bullets only seem to expand back to about 3/4 of their length. So, their CG is much farther away from the force resisting the penetration. For the same angular deflection if they hit a bone or something there will be a larger moment trying to rotate the bullet...'n stuff.... [​IMG]

    Anyway, that's just my theory and I have no experience to back it up.

    But, it's enough for me to just use the 160's until I rebarrel my rifle. That reminds me, if I want to test them by this summer, I had better order them now, eh?