determing twist rate and optimum velocity 7mm bore

Discussion in 'Reloading Forum (All Calibers)' started by crustyrusty, Oct 11, 2019.

  1. crustyrusty

    crustyrusty Silver $$ Contributor

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    hello I presume this has been hashed over so many times but evidently its still a hot topic.. I just put together a 7mm BR rifle with a 1-9 twist Wilson SS 26" #4 contour barrel and is topped off with a new 6-25x50 CV life scope w/sunscreen My bullets of choice is the 100gr HP sierra and 110gr speer hp and the 115gr speer hp and the sierra 120gr sp and the speer 130gr spbt ..and speer 145gr spbt .without a chronograph my velocity's are a guesstemate at 2800fps for the 100, 110, 115, and 2700fps for the 120gr and 130gr... so I have been told that my twist rate is too fast.. so I found a site on the web that calculates twist rate based on the bullet design and length and lo and behold if all I wanted to shoot 100grners a 1-16 is good to go. move to the 110 and it shrinks to 1-15.. 120gr takes it to 1-15 twist ...130grners went to 1-14. 145grners went to 1-12.75 twist.... (i imagine 1-12 would suffice) So overall a 1-12 twist it would seem is what I need as I won't shoot anything heavier than 145gr. has anybody else ran across this para-dime? and if I do shoot this one-in nine twist tube,when will bullet RPM's be so great as to grenade the bullet? or is that so far of velocity wise I dont need to worry about that? or will full power loads overcome any shortfall in the chamber and fly straight? Or should I start saving money right now and upgrade to the 1-12 barrel? the whole reason why I went to the 7MM BR was to shoot light loads in the first place.
     
  2. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

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    Youll never blow up a bullet in a 7br. Your velocity estimates seem kinda high for the heavies- youre approaching 284 light load velocities with a lot smaller case. Not saying it isnt right it just seems high. And according to your intended use your twist may be fine- never know til you try it! I never advocate a universal twist, like a twist that will shoot 52 gr 22 bullets and still shoot 90gr.
     
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  3. AckleymanII

    AckleymanII Silver $$ Contributor

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    Crusty, you are picking the black specs out of the pepper. It will take a while for you to burn out the 7 BR barrel.

    Time for you to go burn some powder, you are good to go.

    Just curious, what is your intended use for this rifle? Freebore to accommodate the 145g maybe an issue. 120g Nosler ballistic tip & the 130g Sierra Match, 130g Speer btsp maybe a good place to start. I would focus on the 110g Speer tnt.

    Our predator calling club had "Big Gun" hunts for coyotes. I shot a 7 Mag loaded with the 115g Speer hp, they did not blow a big hole in coyotes, but the recoil made me feel real good at that stage in my life.

    I have wanted a 7 BR in a rifle for a long time and will watch your progress with great interest. I would start off with 110g Speer tnt for general varmint use, and go 130g speer btsp for deer and hogs. For targets the 130g Sierra MK is going to be hard to beat but the 110g speer tnt may just give it a run for it's money.

    Best of luck!
     
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  4. chromatica

    chromatica

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    There is a fairly intricate formula for computing the Miller Stability Index. It requires the bullet characteristics including BC, as well as atmospheric conditions. I ran the numbers using BC numbers I found at bullet mfr websites. I used educated guesses for bullet lengths for some of these. Using standard atmosphere at sea level here are my results:



    100gr @ 2800 fps Stability Index = 2.104

    110gr @ 2800 fps Stability Index = 1.801

    115gr @ 2800 fps Stability Index = 1.673

    120gr @ 2700 fps Stability Index = 1.539

    130gr @ 2700 fps Stability Index = 1.493

    145gr @ 2700 fps Stability Index = 1.498



    Bullet velocity drops faster than rotational speed so stability always increases as the bullet moves downrange from the muzzle. General guidance is to keep the Stability Factor > 1.5 at the muzzle. All of these scenarios pass the minimum stability test. The 100gr scenario looks mildly overstabilized to me.
     
    Last edited: Oct 12, 2019
  5. crustyrusty

    crustyrusty Silver $$ Contributor

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    well my intention was to have a barrel that shot 100gr thru 145gr and nothing heavier. so I chose the 1-9 twist thinking that was the best "compromise". guess i shoulda looked harder at the information available on the web and made a better informed decision.... and the 145gr will handle almost any animal I want to shoot including black bear(not saying thats a wise decision but hey if it presents itself) If i fail to get what i origialy intended I will re-barrel to a slower twist and move to a new provider of barrel as well... I will go thru Pac-Nor for their barrel if this one doesnt work..... I really do like my 110gr and 115gr bullets i hope to make them sing..
     
  6. crustyrusty

    crustyrusty Silver $$ Contributor

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    those are the same numbers I used to figure twist rate per given bullet weight/length. and thats how I arrived at a twist factor of 1-12 as optimum for those bullets listed...
     
  7. chromatica

    chromatica

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    1-12 will be way too slow. Stability Index for 1-12 with 100gr scenario is 1.184. Your bullet is likely to tumble.
     
  8. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

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    Twist rate is determined by the bullet length not weight. A vld bullet as opposed to a regular spitzer bullet of the same weight takes a different twist. Perfect example are the newer lead free bullets. I have seen some 52gr 22cal bullets that need a 9 twist.
     
  9. geo.ulrich

    geo.ulrich beware of owner Gold $$ Contributor

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    Why would you think that, I have shot 100 gr 7 mm bullets in registered s.r. bench rest matches with 16 and 17 twist.
     
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  10. crustyrusty

    crustyrusty Silver $$ Contributor

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    this is where I went to find my twist recommendation......http://kwk.us/twist.html according to this calculator a 160gr sp takes just a little better than 1-10 twist almost 1-11t actually I did find the 175gr sst's that I had/have required a twist of 1-8.5 no wonder they key holed out of my rifle in low velocity loads
     
    Last edited: Oct 12, 2019
  11. crustyrusty

    crustyrusty Silver $$ Contributor

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    well all I am going by is QL data and this is a sampling of what I have to go by
    Hodgdon BL-C2 106.6 36.8 2.38 2877 97.1 57000 6678 1.086 ! Near Maximum !
    Winchester 748 103.8 35.3 2.29 2875 98.0 57000 6504 1.081 ! Near Maximum !
    Hodgdon CFE223 105.5 37.1 2.41 2873 96.0 57000 6714 1.081 ! Near Maximum !
    Bofors RP11 TZ ~approximation 107.0 32.9 2.13 2863 100.0 56015 5781 1.086 ! Near Maximum !
    Accurate 2520 107.0 35.6 2.31 2856 98.6 56223 6376 1.100 ! Near Maximum !
    Nitrochemie A/S 0300 95.6 31.0 2.01 2849 100.0 57000 5844 1.092 ! Near Maximum !
    Alliant AR-Comp 107.0 32.7 2.12 2849 100.0 54913 5771 1.095 ! Near Maximum !
    all loads for the 120gr sp

    and these loads for the 130gr spbt load
    --------------------------------- -----------------------------------------------------------------
    Hodgdon CFE223 104.0 36.1 2.34 2770 97.0 57000 6634 1.122 ! Near Maximum !
    Hodgdon BL-C2 104.9 35.7 2.31 2768 97.9 57000 6577 1.128 ! Near Maximum !
    Winchester 748 102.2 34.3 2.22 2766 98.7 57000 6386 1.123 ! Near Maximum !
    Alliant AR-Comp 106.3 32.1 2.08 2754 100.0 57000 5627 1.123 ! Near Maximum !
    Bofors RP11 TZ ~approximation 105.7 32.1 2.08 2754 100.0 57000 5627 1.123 ! Near Maximum !
    Accurate 2520 105.6 34.7 2.25 2753 99.3 57000 6255 1.138 ! Near Maximum !
    Hodgdon H4895 106.4 33.2 2.15 2741 97.3 57000 6419 1.138 ! Near Maximum !
    Accurate 2495 107.0 32.7 2.12 2737 100.0 54732 6044 1.175 ! Near Maximum !
    Ramshot Wild Boar 104.6 34.9 2.26 2735 96.6 57000 6413 1.132 ! Near Maximum !
    and this is my sample for the 110gr HP load


    Winchester 748 105.0 36.7 2.38 3005 97.3 57000 6688 1.038 ! Near Maximum !
    Hodgdon CFE223 106.6 38.6 2.50 2998 95.0 57000 6865 1.039 ! Near Maximum !
    Nitrochemie A/S 0300 97.0 32.4 2.10 2992 100.0 57000 6091 1.045 ! Near Maximum !
    Hodgdon BL-C2 107.0 38.0 2.46 2979 95.9 55206 6800 1.056 ! Near Maximum !
    SF033 fl, russ. 5,45x39 100.7 33.5 2.17 2974 100.0 57000 5775 1.055 ! Near Maximum !
    Hodgdon H335 99.7 35.4 2.29 2973 98.7 57000 6423 1.046 ! Near Maximum !
    ADI AR 2219 99.4 33.4 2.16 2973 99.3 57000 6267 1.043 ! Near Maximum !
    Hodgdon H322 105.8 33.4 2.16 2973 99.3 57000 6267 1.043 ! Near Maximum !
    SNPE Vectan SP 10 99.3 34.5 2.24 2968 98.6 57000 6391 1.051 ! Near Maximum !
    Bofors RP11 TZ ~approximation 107.0 33.8 2.19 2968 100.0 53477 5996 1.060 ! Near Maximum !
    PB Clermont PCL 508 (PCL223) 99.2 34.5 2.24 2967 98.6 57000 6381 1.051 ! Near Maximum !
     
  12. crustyrusty

    crustyrusty Silver $$ Contributor

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    yeah I know that that a 100gr bullet is also a short bullet so length and weight run hand in hand until you get to a lead free which run much longer per given weight I dont have much experience with the VLD or lead free bullets I just spent and hour measuring all of the various bullets I have on hand and ran them threw the twist calculator it was eyeopening to say the least.. I found a 8mm 150gr sp that takes a 1-17 twist and a 7mm 175gr sst/eld take a 1-8.5 twist and my 55gr 5.56 takes a 1-12 twist so MY rifles twist rates is 1-9 for my .223,,, 1-9 for my 7mm BR and 1-10 for my 8mm........so many compromises
     
    Last edited: Oct 12, 2019
  13. geo.ulrich

    geo.ulrich beware of owner Gold $$ Contributor

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    Both Greenhill and Miller's formula's are way outdated. Like 50 + years in Greenhills case. Go to JBM or use Mcgyro they are from this era....
     
  14. mikecr

    mikecr

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    ~3,700fps
    It would take a really big case and very long barrel to reach.
     
  15. crustyrusty

    crustyrusty Silver $$ Contributor

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    ok i was just worries thats all i didnt know the RPM threshold that that bullet can withstand before yaw and pitch take over and go Bang!or jacket seperation whichever occurs firstly
     
  16. BenPerfected

    BenPerfected Gold $$ Contributor

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    Take a look at the Berger twist rate calculator. This is a place to start from.
    Ben
     
  17. crustyrusty

    crustyrusty Silver $$ Contributor

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    on the site thatI hi-lighted they made mention of some mathmatical enhancements to those formulas to bring them into the 21'st century and that they also said that JBM could add "refinement to the formulas" they also said "John Knight in England offers a free Win32 executable, WinGyro, which provides a much more sophisticated analysis than done in the calculator here. In comparing results from this calculator to those of WinGyro, it appears that Bowman's correction can be too optimistic regarding the effect that muzzle velocity has on stability. Using a velocity of 1840 fps here (reducing the calculations to that of Greenhill's equation) sometimes gives a better match to WinGyro's results. Another fine tool for estimating twist rates (and ballistic coefficients) is found on the JBM site. so I guess Its a matter of relativity how close to "close" does it matter? I shoot 1-9 now but where will I be if I choose to go to 1-12? I allready know that 1-16-17 works for the 100grner and 1-15 for the 110 and 115gr so I would be compromizing again with a 1-12 just to shoot 145gr btsp?or does the 145gr pill really work @1-13?1-14? I dont know anymore
     
  18. geo.ulrich

    geo.ulrich beware of owner Gold $$ Contributor

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    The more modern formulas take in form factor closer to what we use now, in other words bullet length is not the only thing involved nose length or the length of nose radius , either more or less increases or decreases drag which changes sg...
     
  19. mikecr

    mikecr

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    Why are you so worried about over-twist being a problem? You act like it won't work.
    Also, you could simply pick a bullet you know. It all begins with a bullet,, powders, barrel, etc.
    Do you want to shoot 100gr bullets, or 145gr bullets? Just choose
     
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  20. crustyrusty

    crustyrusty Silver $$ Contributor

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    because I was shocked that my first 5 rounds (100gr) grouped and 1.5inches at 50yards so I tried the 110gr the next 5 grouped 2inches at 50yards i didnt want to tell anybody because iwas so dumbstruck.. so I brought all my loaded ammo home pulled all the bullets and went to my real 7mm BR cases and started all over again this time using a heavier load of 8208 powder (33gr) with 110gr pill so my next 3 shot group was a dime at 50 yards so I strected it to 100 yards and was rewarded with just a vanilla group of 1.5 6 shot ave
     

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