1. Get Gear'd Up! Enter to WIN $1000 in gear!

    Please Click Here for full details and to enter. You will need to be registered and logged in to view the details and to participate.

    Thanks and good luck to everyone

Greenhill or Miller?

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

  1. Brian Albin

    Brian Albin New Member

    Joined:
    Aug 16, 2012
    Messages:
    34
  2. JLA

    JLA Well-Known Member

    Joined:
    Feb 26, 2007
    Messages:
    18,271
    Location:
    Heart Of Texas
    Greenhill works for me. I refuse to argue with 1/4 MOA groups.
  3. Archie

    Archie Member

    Joined:
    Mar 6, 2005
    Messages:
    352
    Location:
    Hastings, Nebraska; the Heartland!
    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

    Joined:
    Feb 26, 2007
    Messages:
    18,271
    Location:
    Heart Of Texas
    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

    Joined:
    Aug 16, 2012
    Messages:
    34
  6. Brian Albin

    Brian Albin New Member

    Joined:
    Aug 16, 2012
    Messages:
    34
    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

    Joined:
    Feb 26, 2007
    Messages:
    18,271
    Location:
    Heart Of Texas
    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

    Joined:
    Dec 20, 2003
    Messages:
    6,693
    Location:
    Hesperia, CA
    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

    Joined:
    Aug 16, 2012
    Messages:
    34
    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

    Joined:
    Dec 20, 2003
    Messages:
    6,693
    Location:
    Hesperia, CA
    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

    Joined:
    Feb 26, 2007
    Messages:
    18,271
    Location:
    Heart Of Texas
    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

    Joined:
    Aug 16, 2012
    Messages:
    34
    Alright. Thanks everybody.
  13. JLA

    JLA Well-Known Member

    Joined:
    Feb 26, 2007
    Messages:
    18,271
    Location:
    Heart Of Texas
    Is that it?? I was getting into this discussion. I dig ballistics and physics.
  14. Brian Albin

    Brian Albin New Member

    Joined:
    Aug 16, 2012
    Messages:
    34
    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

    Joined:
    Dec 20, 2003
    Messages:
    6,693
    Location:
    Hesperia, CA
    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
Similar Threads
Forum Title Date
The Ammo & Reloading Forum Greenhill Formula and Twist....... Sep 6, 2008

Share This Page