Greenhill or Miller?

Discussion in 'The Ammo & Reloading Forum' started by Brian Albin, Aug 28, 2012.

  1. Brian Albin

    Brian Albin New Member

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  2. JLA

    JLA Well-Known Member

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    Greenhill works for me. I refuse to argue with 1/4 MOA groups.
  3. Archie

    Archie Member

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    The Greenhill formula has been around for over one hundred years and seems to be the basis for calculating spin. I just looked at the Miller website and I see no information on how the answer is calculated. Therefore, I cannot give any sort of meaningful answer as to 'preference'.

    As always, the proof of the calculating is in the shooting. But I'd hate to build a custom rifle with custom twist and then find out it almost works.

    From everything I've read, and from all the experimentation I've done, a lot of over spin is better than just a little under spin. When in doubt, twist it faster.
  4. JLA

    JLA Well-Known Member

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    Underspun projectiles will NEVER stabilize. Overspun projectiles will yaw in flight just a little longer before 'going to sleep' and flying true along thier trajectory. The 175 SMKs i shoot from my .308 are overspun by about an inch of twist, and they dont fall asleep until the 150 yd mark. That rifle shoots the exact same 1/2" groups at 200 yds that it does at 100. The explanation as to why it doesnt shoot 1/4 MOA at 100 is due to bullet yaw.
  5. Brian Albin

    Brian Albin New Member

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  6. Brian Albin

    Brian Albin New Member

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    Miller says in his opening Summary “I compare how well such rules fit actual experimental data for 14 bullets at various velocities (40 examples.)”

    “actual experimental data” I could not find that he shot any actual experiments. The only further mention of experimentation I could find is on page 3 the paragraph titled “Sources of Input Data” saying: “These twist rules were tested on 40 examples for 14 bullets from various sources, almost all from Ballistic Research Lab reports. Such experimentally-measured data, which include weight, twist, stability factor, bullet length, and velocity, are very hard to find.”

    That was the only mention I found in this article of experiment having been done. What exactly was done by these Ballistic Research Labs is stated only in the vaguest manner by saying the data include weight, twist, stability factor, bullet length, and velocity.

    In Table 2 we see columns Velocity Experimental and Twist Experimental. This is the data from the Labs. But what is it telling us? When it says Western 180 gr Silvertip 2464 fps and 10 inch twist, What does that mean? Did the lab shoot this bullet at that velocity in a variety of Bbls and find 10.0” to be the best? Or did they go to the hardware store and buy a 30-06 off the shelf, measure it to find it had the same 10” twist they all have as a carry over from the first U.S. Army 30 cal of the 1890s, and chronograph some ammo through it to find that velocity 2464 shot best? Best in what sense? Was this the slowest muzzle velocity which did not show keyholes at some chosen range? Which range? I don’t think I am believing a 180 gr spitzer flat base could not be further slowed and still be stable. So what are these experiments?

    In starting this thread I was hoping for your experimental data.
    Have any of the shooters here tried the same bullet in several Barrels and found one of these electronic calculators to be more representative of your paper target results of accuracy and group size; or when you went from round holes to ovals, than the other calculator?
  7. JLA

    JLA Well-Known Member

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    The greenhill link above is the one i use to do just that. A 180 gr .30 cal needs 10 twist at around 2500 fps.. but the very same bullet at 1000 fps needs a 7. Thats why the .300AAC is manufactured with such fast twist barrels. Its not a particular twist rate bullets need, its a particular minimum RPM. and RPM is a function of rifling pitch X Velocity
  8. LDBennett

    LDBennett Well-Known Member

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    It seems that the Miller twist formula is the answer to a non question. The Greenhill formula has been used for over a hundred years on all caliber rifles and guns (like ships guns) with success. Why would a person need another calculation by someone else that does not agree? Just because a formula is more complex does not mean it is better or even right. Experimental data tells the story and if Miiller has none then I say throw it out.

    I totally agree with the RPM comment by JLA:

    "Its not a particular twist rate bullets need, its a particular minimum RPM. and RPM is a function of rifling pitch X Velocity"

    LDBennett
  9. Brian Albin

    Brian Albin New Member

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    How do you know?
    That is what it calculates to. But what I am asking is: Does anyone challenge this black powder era calculator to see if it is asking for more twist than smokeless velocity bullets need; or has industry and all become trapped by tradition?
  10. LDBennett

    LDBennett Well-Known Member

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    Brian Albin:

    The answers to your questions are int Wikipedia:

    http://en.wikipedia.org/wiki/Twist_rate#Twist_rate

    The barrel maker LIJLA has a series of articles on barrels you might find interesting. While they may use a rule of thumb like the Greenhill formula, their barrel twist rates are base on experience and modify any initial calculated twist rate.

    http://riflebarrels.com/faq_lilja_rifle_barrels.htm#twist rates

    In general todays manufacturers select barrel twist rates that reflect decades of experience. Today's bench rest barrel maker, like LILJA, probably know what works best. You could do worse then to just select a twist rate from the LILJA articles.

    LDBennett
  11. JLA

    JLA Well-Known Member

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    I know because Ive tested it.. Extensively. Go ahead. grab a can of trailboss and a box of factory ammo and a .243 winchester with some 100 gr bullets. run them thru a 9 twist barrel at full power (2800-2900 fps) and see that they shoot great. but fill the case with trailboss and run the same bullet in the same gun at around 1200 or so and it will keyhole at 50 yards. Seems pretty open and shut to me. Only difference being the amount of spin imparted to the bullet. again via velocity X rifling pitch.

    Less velocity means you need a shorter bullet (shorter usually means lighter) that doesnt need the RPM range of the heavier bulets to be stable in flight.
    Last edited: Aug 31, 2012
  12. Brian Albin

    Brian Albin New Member

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    Alright. Thanks everybody.
  13. JLA

    JLA Well-Known Member

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    Is that it?? I was getting into this discussion. I dig ballistics and physics.
  14. Brian Albin

    Brian Albin New Member

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    Yeah, but I aint learnin nuthin.
    You guys don't have any more money than I have. I thought someone here would have experimented with rebarreling his rifle several times; say a 30 cal with 10, 12, 13, 14 & 15 inch rifling and shooting maybe the 155 gr Palma bullet (or any other) to see which twist shot best.
    As Sherlock Holmes says, "I want data".
  15. LDBennett

    LDBennett Well-Known Member

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    Brian Albin:

    Did you read the Lilja link I gave you. They cover twist rates pretty thoroughly. And they do talk about different twist rates for specific bullets.

    Do remember that it is all about the RPM at which the bullet is rotating. And that is not only determine by the twist rate but the velocity too. It would make more sense to compare different RPMs then convert that into twist rate at the velocity of your chosen load.

    Also don't under estimate experience. Some of the better performing bullets have been around a long, long time and others through the years have rung out what twist rate works best. Manufacturers and re-barrelers listen to their customers and I'd bet the popular twist rates for particular calibers are pretty close to the best you can do for that bullet at normal velocities.

    LDBennett
  16. JLA

    JLA Well-Known Member

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    You arent learning because you arent deducting the info youre being provided.

    I have not rebarreld my rifles in that manner, that would be greatly cost prohibitive. But I have tested the theory the very same way by reducing and increasing muzzle velocity using the same barrel and twist rate.

    Ill try to break it down here. You are wanting someone to rebarrel a given rifle action with several progressively slower twist barrels all the while testing the same ammo, thus keeping the bullet weight and the velocity the same throughout.

    Again ill state that bullets require a specific minimum RPM range and RPM is a function of velocity X rifling pitch.

    Shooting a 155 gr .308 load at full velocity around 2900 fps thru progressively slower twist barrels will change the RPM imparted to the bullet the very same way reducing the velocity with reduced loads thru the same barrel over and again. The end result is a spun bullet regardless of the methods and tools youve used to get it there.

    I have found, repeatedly and consistently that once you fall below a bullets required minimum RPM range it will tumble and keyhole on target. You determine this requirement using the greenhill formula by plugging in the projectiles length and diameter along with muzzle velocity and specific gravity figures into the greenhill equation.. and it has been working for me for a little over a decade now.

    Basically what I do is determine what twist rate a particular rifle has, then i determine the average velocity i can expect from the chambering (refer to reloading manuals for this figure) and then i use the greenhill formula to chart a list of suitable projectiles that rifle barrel should shoot well. And if I want to get bullet specific, i will use the greenhill formula along with the rifles twist rate to determine a velocity minimum for THAT bullet.

    The information has been priovided throughout this thread, you just have to understand the concept well enough to deduct what you need from it.
  17. Brian Albin

    Brian Albin New Member

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    I did Dan’s recommendations on his Q&A page. Thanks LD.

    I already knew that bullet stability is measured in rpm, what I am asking is which calculator best expressed how much rpm we need.
  18. JLA

    JLA Well-Known Member

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    And ive been saying it all along.. Greenhill.
  19. Archie

    Archie Member

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    JLA, sorry to differ with you, but it isn't just RPM.

    I haven't rebarreled a rifle several times, but I have shot different length (therefore weight, shape and ballistic coefficient) from the same rifle. A Remington 700 in .22-250 Remington, to be exact. The rifle has a 1 in 12 twist and a 26" barrel.

    Here's the salient point for this discussion: It will shoot bullets up to a certain length very well. Once over that length, the bullets will not stabilize.

    Looking at my immediate notes, the Sierra 69 grain HPBT simply will not stabilize. As fast I as could throw them (I was pushing them pretty hard) the bullets will not stabilize. Turns out, the faster the bullet goes, the greater air resistance causes them to yaw and destabilize more. That increased air resistance more than makes up for the additional RPM.

    However, shooting the Speer 70 grain RNSP gives good results. That is a slightly shorter bullet and stabilizes in the 1-12 spin of the rifle. I do not have the bullet lengths at hand.

    So you are correct in a limited manner. A bullet requires [that] amount of RPM based on ballistic coefficient. But it isn't as simple as [this] amount of spin will stabilize [that] bullet in any and all conditions.

    I am just a bit put off by this. That 69 grain HPBT at .22-250 velocities - if stabilized - will go about forever.
  20. Brian Albin

    Brian Albin New Member

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    Hi Archie,
    Using bullet length.895 from the new forum length list http://www.thefirearmsforum.com/showthread.php?t=112533 the Miller calculator http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi shows stability factor 1.0 (the minimum edge of stability) is achieved at 70 degrees and 30 inches mercury if muzzle velocity is 3200 fps, but the bullet is unstable at 3100.

    Or if the temperature came down to 60 degrees, even 3300 would be unstable; unless the Barometer came down to 29 inches in which case the flight would again be stable.
    In other words, that bullet is on the edge of stability in a 12 inch twist according to Miller.
    So that’s a result. A useful comment on the accuracy or lack of it in this new calculator from Miller.
    Thank you, for the data.
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