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  1. #1
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    Default Insight into Gyroscopic Bullet Stability and Velocity

    Hi All

    I am looking to garner some information relating to a trend I am seeing in the behavior of my hunting rifle when using different loads.

    I have been spending a bit of time recently doing some load development for my model 94 Winchester in .375WIN (30-30 WIN necked up to .377 caliber). I am looking to push the loads down just enough to get sub sonic, around 1100 fps. My original loading was 33-34 grains of S335 giving the 265-270 grain cast bullets a velocity of around 1650-1700 fps. I am well aware that these bullets are a bit heavier than what the design intended, noting the Winchester has a 20" barrel with 1:12 twist.

    As I took the load down incrementally to 28 grains of S335, yielding velocities of just over 1200 fps, using the same cast 265 grain bullet from the same alloy mixture with the same gas checks. I noticed a remarkable improvement in accuracy. Groupings closed down from over 3" to under 1.6" at 100 yards. This grouping behavior has been consistent and was not a "one off".

    So the question I have: is there a relationship between bullet velocity and gyroscopic stability when the rifle twist rate is constant. In other words, does a reduction in bullet velocity assist in stabilizing heavy bullets? Has anyone experienced this with a more mainstream caliber like .308 or 30-06 FMJ's.

    Please note: I am also aware that there is a multitude of other variables which can influence accuracy in this situation. I do not test the hardness of my alloy, instead I use a fixed proportion of Linotype and lead alloy, this could be a factor as velocity increases. I also feel that I could "sub-consciously" flinch at the recoil when the energy moves toward 1800-1900 foot pounds, given the light weight of the Model 94 and it's relatively high bore axis.

    Once again your insight is appreciated.

    Regards

  2. #2
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    Default Re: Insight into Gyroscopic Bullet Stability and Velocity

    Read up about barrel harmonics, internal, transitional and external ballistics as well. It's very possible that at the reduced load, you hit an accuracy "node" as they're called in your rifle, making the bullets behave better on their way to the target.

    To sum it up in the beginner way: A rifle's barrel isn't stationary during shooting. It wobbles like a wet noodle. At specific loads you can make it wobble in a very similar way shot-to-shot, increasing it's precision (it's ability to place shots in the same place over and over again) with that load. That load and wobble combination is called an accuracy node.

  3. #3
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    Default Re: Insight into Gyroscopic Bullet Stability and Velocity

    Gyroscopic stability actually increases a little as velocity increases, all else being equal. So no, your accuracy improvement is not related to gyroscopic stability. I have found that the 1:12" twist rate of my 375 H&H's barrel will stabilise a 270 gn flat nosed cast lead bullet at 1500 - 1700 fps just fine. It stabilises much longer bullets as well, but I have not calculated the gyroscopic stability factor (Sg) yet. For sufficient stability from normal rifle barrels, a minimum Sg value of 1,5 is recommended. For subsonic loads using relatively long bullets, I see that a higher Sg is preferred. Then there are the muzzle-loader shooters, who often go to a significantly lower Sg with very short conical style bullets.

    I would suggest that you experiment with your particular rifle and bullet and find a combination that works for you. You seem to be on the right path already.

    My guess is that the improvement in accuracy you experienced has to do with bullet hardness, fit to barrel, harmonics and other variables, not Sg.

    Controlling your flinch is one of the arts every shooter has to master. You could ask a friend to load the rifle, and sometimes to load a dummy round or empty case, without you knowing which. Then you will easily see whether you are flinching or not.

  4. #4
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    Default Re: Insight into Gyroscopic Bullet Stability and Velocity

    Your increase in accuracy with the lead bullets at lower velocity indicates the hardness of the lead is more suited to lower presure.

    When the lead is too soft for the velocity / pressure, it skips the rifling and you get a shotgun pattern. This is with or without gas checks.

    The best accuracy with high velocity lead bullets is allegedly achieved with paper patching, irespective of lead hardness - my project for 2020.

  5. #5

    Default Re: Insight into Gyroscopic Bullet Stability and Velocity

    You can also be experiencing the effects of transonic flight of the bullet (from supersonic to subsonic).
    You are operating at close to the same speeds as .22LR subsonic ammo and they are normally more accurate that high velocity ammo.
    I’m thinking that the bullet is now stable at the target and has gone through the transonic stage of flight


    Sent from my iPhone using Tapatalk

  6. #6
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    Default Re: Insight into Gyroscopic Bullet Stability and Velocity

    The mathematics of stability of a spinning projectile is a second or third order partial differential equation, if my memory serves me correctly.
    Obviously fairly heavy going to do the analysis.
    The higher rotation rate of the higher velocity bullet will tend to stabilize it more.
    In my opinion, 414gates is correct: your lead is stripping in the rifling. Try a slightly harder alloy.

  7. #7
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    Default Re: Insight into Gyroscopic Bullet Stability and Velocity

    Pre64 should step in here now, but he told me there are Cob in the river and he is sleeping on the water tonight, nutter! It’s raining.
    When he and I were doing the 458x2" we found the same thing, a bullet that shot well at our intended speeds also shot well again at sub sonic speeds. The grouping achieved at sub sonic was almost silly, 3 touching/cutting holes from a 458 Cal is quiet something. Pre64 got bored after the 458 experience and started fiddling with something else, and I decided to turn my attention to the .308 which I also got to be an incredible subsonic weapon. Again the bullet weight that was best in my 308 at hunting velocity was also brilliant at subsonic, perhaps the mention of accuracy nodes has some merit? But I am going to stick neck out and state my belief. The subsonic round is accurate because of the lack of things that can make it inaccurate. The lack of big bang, the lack of big time barrel flexing, the lack of stress on bullet, the lack of pressure waves - just the lack of everything that needs to be controlled measured, timed and overcome. The bullet is launched sedately without awaking the Gods, meets no real laws of physics along the way and merely pass's along undisturbed to target.
    Sure this will not happen if the twist is way out, or if the bullet or barrel is over under sized etc., but if as said before you have an accurate hunting round and you work it down to subsonic, there is a subsonic spot and its very accurate, better than the sonic spot - node.
    Hope I am making sense.

    Regarding the higher velocity issue, the advice given regarding bullet hardness if cast bullets are used is my answer as well.
    Check your barrel after a few shots, the leading should be quiet visible.

  8. #8
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    Default Re: Insight into Gyroscopic Bullet Stability and Velocity

    What does the bullet look like?

  9. #9
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    Default Re: Insight into Gyroscopic Bullet Stability and Velocity

    Quote Originally Posted by Messor View Post
    What does the bullet look like?
    My guess is the SWC style bullet as cast from the fairly common RCBS and Lyman moulds, with 4 lube grooves and GC.

    Quote Originally Posted by treeman View Post
    ...
    Check your barrel after a few shots, the leading should be quiet visible.
    I find that when using GC bullets, the bullet may strip it's rifling, but the barrel stays clean, since the GC scrapes all the shaved lead from the barrel. The GC can turn on the bullet base, as it is only lightly crimped on (and springs back a little), so it usually does not strip it's rifling along with the too soft bullet.

  10. #10
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    Default Re: Insight into Gyroscopic Bullet Stability and Velocity

    Hi All

    Thanks for all the valuable insight, based on much of the information conveyed and another test at the range this weekend I have come to the following conclusions:

    No.1: I am going to be investing in a lead hardness tester, namely the Lee product. It gets some decent reviews online and is specific to bullet casting rather than a more industrial system giving readings in "Brinell" units. I see it as a valuable tool going forward as the next application, currently in the CFR pipeline, will be getting my alloy to spec such that I can cast 120 grain 9mm bullets for my PCC where I will expect to see "higher than pistol velocities".

    No.2: After further testing now lighter 26 grain S335 loads, I have re-affirmed the accuracy improvement seen at a touch under 1100fps. Although I wasn't shooting off a bench, rather seated with the aid of a tripod, my groupings are consistently under 2MOA with which I am more than happy. Going forward, I am going to focus on the desired result of this Model 94 build, that being the addition of a suitable non-telescoping suppressor. Given the intended purpose of this rifle, taking "affordable" game animals at reasonable distances and noting the weight of the projectile, I can still achieve 750 foot pounds at the muzzle whilst the platform remains accurate, light, comfortable and fun to shoot without having to worry too much about effects of velocity on projectile hardness and sub sonic transition etc.

    In terms of the bullet: They are cast using an RCBS 250grain (actual 260-265 grain) mold, sized to .377, they have have 4 gas rings and space for the GC (Hornady brand). The bullets are of a flat nose profile. And yes, I too have never had any lead fouling in the Winchester barrel whilst using the GC's, this is confirmed as I did make some 9mm bullets from the same batch of alloy and yes there were indeed traces of lead left in the barrel of my handgun after shooting 40 or so rounds.

    Thanks again for the valued insight.

    Regards

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