Gain twist barrels

Brutas

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I was e-mailing a customer in the U.K. who had some questions large increases in twist rate. So, I thought I'd share.

We have done a few barrels with a large increase in twist. From the reports we received, they shoot fine. However, I generally do not recommend it. We have found that there is no statistically significant reduction in pressure or increase in velocity with the gain twist barrels (They won't be slower than a straight twist barrel, but any gains likely represent nominal differences that exist from barrel to barrel). The main benefit of the gain twist appears to be a reduction in vibration and harmonics. With the gain twist, the bullet is always under constant resistance from the increasing twist rate. This resistance acts like a mechanically choke and provides a better gas seal. This resistance also seems to extend the pressure curve out further, keeping the barrel stretched forward, reducing vibration and limiting the harmonic effects. This reduction in vibration can provide better accuracy. By keeping the amount of increase in twist rate relatively small, the amount of distortion to the bullet jacket is kept at a minimum. In a large increase in twist, the amount of distortion to the jacket could be detrimental to accuracy and increase the possibility of jacket failure. At this point in time, we have found the best result using .75 – 1.00 inch increase in twist rate, depending of barrel length. Some shooters have even been experimenting with an increase as small as .10 of an inch on a 22 inch barrel (the results are interesting, but not enough data has been gathered for a conclusion yet).
 

NukeMMC

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  • Mar 3, 2009
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    Re: Gain twist barrels

    Schuemann has been doing gain twist on their AET barrels for a while. I had one in a Caspian 38super IPSC pistol for a while. That pistol was ACCURATE. Of course, pistols and rifles are apples and oranges, so some of their info may not apply to rifles, but it's interesting anyway.
     

    Wolvenhaven

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    Re: Gain twist barrels

    I'm new to precision shooting, and didn't know this existed, but I was thinking a few days ago "I wonder..." about exactly this. Thanks for the information, it's really interesting.
     

    Graham

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    Re: Gain twist barrels

    I went with a 9.75 to 8.9 for my .284 with 180's. What gain-twist would you recommend for a 6.5 Creed with 140 AMAX factory? Are there any combinations in 6.5 that have given good known results?
     

    BOLTRIPPER

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    Re: Gain twist barrels

    Boltripper always thought that gain twist work elegantly for bolt gun applications, famous bukkake champion had one ....and it shot bungholes,,,,but that was in a bolt gun.....the creedmoor was designed to run in gassers, and somehow i think doing the twistie thing around the area of the blow hole is bad ju-ju



    just sayin
     

    xNF_9

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    Re: Gain twist barrels

    This sounds like a very interesting topic. Would a major manufacturer do a twist like this for you or can you only get a straight twist.
     

    shoot4fun

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    Re: Gain twist barrels

    6.5CM, as you say, may have been desgined for gassers but it works devinely in a bolt gun too.
    S&W uses gain twist in the big X-Frame 460 to reduce pressure. I am not sure how much difference it makes though.
    I think I'll stick with conventional twist barrels.
     

    K. Johns

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    Re: Gain twist barrels

    If a bullet hit lands and starts spinning.

    4" in its spinning at 200,000 rotations per minute(lets just say)

    If you are able to continue an even or increasing gain in twist all the way up to the crown....

    and it leaves that crown at 240,000 rotations per minute

    Would it be spinning faster than 240,000 rotations per minute after leaving the barrel?


    Maybe this will benefit people at long range, using a twist normally not fast enough for a given bullet. If its a gain twist, maybe you can keep the higher velocity and lower pressure of the slower twist, but be able to shoot the heavy bullets because it continues to gain rotational speed after exiting the muzzle. This would keep them stable further.

    Think thats happening?



     

    300sniper

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    Re: Gain twist barrels

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Keith at PCR</div><div class="ubbcode-body">If a bullet hit lands and starts spinning.

    4" in its spinning at 200,000 rotations per minute(lets just say)

    If you are able to continue an even or increasing gain in twist all the way up to the crown....

    and it leaves that crown at 240,000 rotations per minute

    <span style="color: #FF0000">Would it be spinning faster than 240,000 rotations per minute after leaving the barrel?</span></div></div>

    nope.
     

    K. Johns

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    Re: Gain twist barrels

    If it was gaining rpm while in the barrel, and kept gaining until it left, it wouldn't increase at all when it left? I know velocity isn't increasing anymore but velocity doesn't increase much in the last few inches anyways. Hell if you made a 34" barrel 308 what would happen in the last 6" velocity wise? Nothing? If it was a gain twist it wouldn't increase rpm at all when it left? Why would the increase suddenly stop once its let free?

     

    300sniper

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    Re: Gain twist barrels

    they need to bring back "mr. wizard's world"
    grin.gif
     

    E. Bryant

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    Re: Gain twist barrels

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Keith at PCR</div><div class="ubbcode-body"> If it was a gain twist it wouldn't increase rpm at all when it left?? Why would the increase suddenly stop once its let free?</div></div>

    Acceleration (linear or rotary) requires a force. Once that force is removed, the acceleration stops. In the case of a gain-twist barrel, when the bullet leaves the barrel, the force that was causing the acceleration ceases to exist. Contrary to popular belief, there is no sort of "momentum" or anything that would cause the acceleration to continue.
     

    300sniper

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    Re: Gain twist barrels

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Keith at PCR</div><div class="ubbcode-body">I figured the force was the accelerating momentum in that rotational direction </div></div>

    don't confuse acceleration with momentum.
     

    Arbiter

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    Re: Gain twist barrels

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Keith at PCR</div><div class="ubbcode-body">I figured the force was the accelerating momentum in that rotational direction</div></div>

    Remember those merry-go-rounds at the playground?

    If you stick another unwitting 3rd grader onto one, and spun it up to vomit-inducing speeds before letting go, did it keep going faster and faster until he went tumbling off the side?

    A spinning bullet, just like any accelerating object, is only going to gain acceleration so long as something's forcing it to go faster. Take away that force, and it's going to slow down (unless perfectly frictionless in a vacuum, etc).
     

    K. Johns

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    Re: Gain twist barrels

    When using something like a car accelerating as an example it makes sense that acceleration in spin would stop as soon as the force stopped.

    But since a bullet accelerates rpm amazingly fast, I dunno it just seemed different.

    But ok I see how it wouldnt.
     

    Sendero_Man

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    Re: Gain twist barrels

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Graham</div><div class="ubbcode-body">Bartlein will do whatever you want. </div></div>

    Here is the results of my Bartlein Gain Twist..

    starts 1-9 ends 1-8. GAP 6mm Crusader

    5 shot group... I have many targets that look just like this.

    downsized_1224101540b.jpg


    I know when this one goes, I will get another spun on !

    I would highly recommend them to anyone !!!

    cool.gif
     

    Overflow10

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    Re: Gain twist barrels

    Interesting topic. I assume in general the increase in spin = good for heavily bullet caused by a more stable flight.

    However is there any accuracy increases by minimizing the damage/changes to the bullet jacket? The OP mentioned avoiding damage to the jacket when there is too much gain. But is there jacket "damage" caused by a normal straight twist barrel? So instead of a straight twist at 1:9 could a barrel that started at 1:10 and ended at 1:9 actually be more accurate than the straight barrel? Less distortion would occur at 1:10 and it gets ramped up to 1:9?

    Did I just butcher that question? LOL.
     

    K. Johns

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    Re: Gain twist barrels

    I think no matter what there will be more jacket deformation with a gain than with straight.
    But not enough to matter in some cases.


    Im sure I'll try it one day. My next tube will definitely be a a Bartlein
     

    xNF_9

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    Re: Gain twist barrels

    Sendro could you say that a start 1-11 to a 1-10 be good for a 308? Just a question.
     

    nervous

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    Re: Gain twist barrels

    I have a Bartlein gain-twist barrel, it is a 308win, it starts 1-11 and ends 32" later with 1-10. This barrel is mated to a Barnard P action that is in an Eliseo B-1 stock. I do all my load development at 300 yards. The best load I have been able come up with so far was 1moa, this was with a Berger 210gr moving 2,550fps. I hope I can find the sweet-spot soon.

    Nervous
     

    nervous

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    Re: Gain twist barrels

    It will be a couple of weeks before I can get the tube-gun back the the range on the farm, I have to first finish filling the greenhouse with Holly, Clethra and Magnolia cuttings.

    Nervous
     

    nervous

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    Re: Gain twist barrels

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Nervous</div><div class="ubbcode-body">I have a Bartlein gain-twist barrel, it is a 308win, it starts 1-11 and ends 32" later with 1-10. This barrel is mated to a Barnard P action that is in an Eliseo B-1 stock. I do all my load development at 300 yards. The best load I have been able come up with so far was 1moa, this was with a Berger 210gr moving 2,550fps. I hope I can find the sweet-spot soon.

    Nervous </div></div>

    46.5gr RL-17, 2,600fps, CCI-BR2, Berger 210gr LRBT .010 off the rifling, elevation spread .995", windage spread .375". @ 300yards. I will try the same load again in a few days just to make sure it was not a fluke.

    Nervous
     

    dbooksta

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    Re: Gain twist barrels

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Overflow10</div><div class="ubbcode-body">However is there any accuracy increases by minimizing the damage/changes to the bullet jacket? The OP mentioned avoiding damage to the jacket when there is too much gain. But is there jacket "damage" caused by a normal straight twist barrel?</div></div>

    Yes: you can often see "skid" marks on a recovered bullet from where it first hits the rifling, before it has engraved enough to start rotating. Presumably this is one of the benefits of seating bullets into the lands: The bullet doesn't get a running start on the rifling; it encounters the rifling at low or zero velocities so there's no skid.

    Frankly, I'm surprised anybody thought you could use fixed-rate rifling without seating into the lands and not have all sorts of problems from the attempts at instantaneous rotational acceleration. But evidently in practice it's not that big a deal.

    Still, I would expect that holding all else equal a progressive-twist barrel would suffer less harmonic impulse (and distortions) and foul less than a fixed-twist barrel with the same final twist rate.
     

    culdee

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    Re: Gain twist barrels

    last time i heard about gain twist rifling was when I shot with the single shot folks using cast bullets. Harry Pope was the guru of gain twist bbls in the "olden days." As for seating the bullet out into the rifling for curiosity you might want to read up on "breech-seating" cast bullets. Its a unique way of shooting.