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Thread: bullet stability question

  1. #1
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    bullet stability question

    So I'm deep into some 50-cal work. Building 2 50BMG Match Rifles, some hunting guns and re-opening my old 50 Fart project.

    I've got 50cal barrels and cutoffs lying thick on the ground

    Back when I spec'd the original 50 Fart, a subsonic 50 using 10"-16" barrels to push 750-924gr bullets I ordered barrels at 7.5 twist. I spent a couple yrs experimenting with bullets..... cast, patched, lathe-turned and conventional.

    Regarding bullets the Fart project has reached an apotheosis, a zenith, a point where the experimentation re bullets is OVER.


    I've now got short-stubs in 7 1/2, 13 and 15 twist..... and cannot access good opinions as to stability VS velocity.


    As in "will a 50BMG barrel stabilize a 750 @ 1050fps???"


    Can anyone here opine re slowest as in loosest viable twist rate for the 750 A-Max at 1050fps?

  2. #2
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    Quote Originally Posted by alinwa View Post
    So I'm deep into some 50-cal work. Building 2 50BMG Match Rifles, some hunting guns and re-opening my old 50 Fart project.

    I've got 50cal barrels and cutoffs lying thick on the ground

    Back when I spec'd the original 50 Fart, a subsonic 50 using 10"-16" barrels to push 750-924gr bullets I ordered barrels at 7.5 twist. I spent a couple yrs experimenting with bullets..... cast, patched, lathe-turned and conventional.

    Regarding bullets the Fart project has reached an apotheosis, a zenith, a point where the experimentation re bullets is OVER.


    I've now got short-stubs in 7 1/2, 13 and 15 twist..... and cannot access good opinions as to stability VS velocity.


    As in "will a 50BMG barrel stabilize a 750 @ 1050fps???"


    Can anyone here opine re slowest as in loosest viable twist rate for the 750 A-Max at 1050fps?
    Here's an online calculator. Hope it helps.
    https://www.omnicalculator.com/physics/twist-rate

  3. #3
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    That's about the same as a .500 / .510 Whisper...

    I'm not sure what a typical twist for such is, but you might find some answers at the SSK website.

    GsT

  4. #4
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    Slick project Al. Dad and I have worked with .50 BMGs since 1991. Never tried to shoot one that slow, but if I had to guess on twist rate, something on the order of 1:9". But again, that's just a guess.

    Hats off to you for tinkering with the BMG. Not many folks take the plunge. Either due to component cost or the muzzle blast of shooting them. My dad built his first in 1991 on one of his actions. It was a large shell-holder design. The whole gun weighs 55 pounds. Ten years later he built a 25 lb version. We were shooting it one day and I decided to see how well his brake worked. So I unscrewed it, let go a 750 Speer over a full charge of 8700, and watched my ear muffs fly off and land a few feet behind the bench. Back on went the brake.

    We almost built a third action when I wanted a .50 BMG in the mid-2000's. I chose the quick route however and bought a State Arms. This one scales right at 42 lbs and recoils like a 12 gauge:



    Having worked with BMG's, I'm sure you're aware of their accuracy potential. My State Arms is cut with a hybrid chamber and I use Lapua brass. 1/4 MOA is possible, which surprises a lot of people. But a well set-up .50 BMG can do it. Mine loves 215 grs of H50BMG with Hornady's 750 AMAX or 245 grs of US869 with the Barnes 800 gr solid bore rider. Please keep us posted as your project progresses.....I'm all ears.

    -Lee
    www.singleactions.com
    Last edited by Lee Martin; 07-21-2022 at 03:48 PM.

  5. #5
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    Quote Originally Posted by Lee Martin View Post
    ...We were shooting it one day and I decided to see how well his brake worked. So I unscrewed it, let go a 750 Speer over a full charge of 8700, and watched my ear muffs fly off and land a few feet behind the bench. Back on went the brake...
    Recall hearing something on shooting .50 BMG rifles. FCSA has a rule on shooting rifles in a match, muzzle brakes MUST be attached to the rifle. Think this a SAFETY rule, which cannot be ignored. Do know an acquaintance (SEAL sniper) tried shooting a .50 BMG rifle minus the brake (1980s or early 1990s), after the first shot, he was medivaced due to broken molars. No joke people, this is SERIOUS. DO NOT shoot a .50 BMG rifle minus the brake, as the recoil is dangerous - potentially LETHAL.

  6. #6
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    Quote Originally Posted by mwezell View Post
    Here's an online calculator. Hope it helps.
    https://www.omnicalculator.com/physics/twist-rate
    I'm not sure if it helped or not

    I got three different answers.

    One said "this bullet is stable"

    "corrected parameters twist rate 12.527"

    And one said "corrected twist .7679/turn"

    I know all the parameters but not sure about "pressure"...... all sorts of ways to measure barometric pressure......I plugged in 14lb

    Bullet is 2.555" long

  7. #7
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    Quote Originally Posted by Lee Martin View Post
    ....... Please keep us posted as your project progresses.....I'm all ears.

    -Lee
    www.singleactions.com
    If you really are interested I'm documenting the entire project on youtube here https://www.youtube.com/watch?v=wED5GAULMVM

    At this point in time I'm pretty wrapped up in using the 50BMG to illustrate/prove/validate many of my beliefs about accuracy. I set myself a challenge with three parameters....

    3 parameters;
    #1, 15lb
    #2, 1/4moa
    #3, recoil "like an '06"

    "Life is short, then you die" ...... so I'm skipping a whole bunch of interim steps and going straight for the moon

    Meanwhile, I'm at a dead full STOP on the subsonic Fart cuz I can't control the recoil. But I might just build a heavy one, like a 15-pounder just because I'm in the mood

  8. #8
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    Quote Originally Posted by alinwa View Post
    I know all the parameters but not sure about "pressure"...... all sorts of ways to measure barometric pressure......I plugged in 14lb
    Normally, this is "station pressure", which is measured using a precision barometer. DO NOT use "pressure" as reported on the TV news - it is local pressure, corrected to sea level values - this is NOT station pressure. Short of using a precision barometer, try calling a nearby FAA facility, as they do report station pressures. NOTE: FAA pressure readings for local runways are best, as knowing and using the station pressure for the runway is important for airplanes landing there. (Oh, look - we've landed in Vancouver, WA, and the barometer we set in Denver, CO says we're 10 feet lower in altitude from the runway's value! )

    Actually, there is a calculation for determining if a projectile is gyroscopically stable - the value of interest is known as a "stability factor", and the desired values are 1.5 or above. Below 1.5, and the projectile may or may not be gyroscopically stable. Have seen two versions of this calculation, with the calculus-based version (integration required) being the more reliable one. (It is probably also the more annoying one to use.) The algebraic version is much easier to use, but results will not be as precise. Not sure how to calculate "Dynamic stability", but both Sg and Sd need to be "stable" for the bullet to be "stable" in flight. The statement "friends don't let friends shoot Sierra 168s (2200 model bullet) at 1000 yards from .308 Winchester rifles" is due to the dynamic instability of the 168 in the transsonic zone (Mach 0.8 to 1.2). At a MV below 2700 fps, think this occurs between 900 and 1000 yards. Believe the (now-discontinued) "short boattail" Sierra 180 also had this problem.

  9. #9
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    Al, If you send the following data I will run through program I use. bullet length, nose length, meplat dia. rough dia. , bt length and base dia. rough dia. altitude where shot otherwise it will calculate at sea level......

  10. #10
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    Quote Originally Posted by geo.ulrich View Post
    Al, If you send the following data I will run through program I use. bullet length, nose length, meplat dia. rough dia. , bt length and base dia. rough dia. altitude where shot otherwise it will calculate at sea level......
    length-2.555
    meplat- (lathe rurned AL tip) we'll call it .060 diameter, .030 radius
    bullet diameter-.510
    bt length- .3
    base diameter .37

    altitude 1,080MSL
    humidity generally 60%

  11. #11
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    Quote Originally Posted by Asa Yam View Post
    Recall hearing something on shooting .50 BMG rifles. FCSA has a rule on shooting rifles in a match, muzzle brakes MUST be attached to the rifle. Think this a SAFETY rule, which cannot be ignored. Do know an acquaintance (SEAL sniper) tried shooting a .50 BMG rifle minus the brake (1980s or early 1990s), after the first shot, he was medivaced due to broken molars. No joke people, this is SERIOUS. DO NOT shoot a .50 BMG rifle minus the brake, as the recoil is dangerous - potentially LETHAL.
    The guns I'm building will hurt you very badly if fired without the brake.

    One of the things I've recently filmed (I don't think it's released yet) is an experiment with clamp-on brakes, one on a 16lb gun and one on a 22lb gun...... fired only on the recoil sled. The same 16lb gun I fire in the rain with a string.

    5th shot the brake came off.

    It broke the recoil sled.

    In several upcoming episodes I approach this.

    IMO a 50BMG under 30lb will hurt you BADLY if fired without a brake.

    A 40 pounder will maybe let you walk away.

  12. #12
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    Al, Using the numbers you provided I get 14.2 @ 1116 f.p.s. (mach1) You didn't include nose length so I ran from a 7- 10 tangent ogive. twist rate stayed the same through out ...g7 b.c. .530

  13. #13
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    .50

    Quote Originally Posted by alinwa View Post
    length-2.555
    meplat- (lathe rurned AL tip) we'll call it .060 diameter, .030 radius
    bullet diameter-.510
    bt length- .3
    base diameter .37

    altitude 1,080MSL
    humidity generally 60%


    Alinwa -

    Howdy !

    Thinking out loud on your .50 subsonic......

    Let's say you have a stability factor already calc'd for a known/ proven load for .50BMG that uses the same bullet you want to use in the .50 subsonic.
    And, you also know pretty much how far out you want to apply the bullet, shot from the .50 subsonic case.

    You could run a sample 1,050 Mv figure ( plus pertinent bullet data ) through a ballistics calculator, and input/re-input varying twist rates; until a stability factor number matching the proven .50BMG load is arrived at. And again, you'd decide on how far out you want to apply that bullet. This should give you a fairly close idea on the necessary twist rate for use w/ the subsonic wildcat w/ the bullet you mentioned.


    With regards,
    357Mag

  14. #14
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    Based on these replies I think I'll not waste time using the 15" twisters for the subsonic project.....

    Thanks!!

  15. #15
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    Lightbulb An idea.........

    A trip to the JBM Ballistics website may help.

    I haven't used it in years........but it still may be there.

    Just a thought,
    Kevin

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