Bullet ''spin''

This is a discussion on Bullet ''spin'' within the General Firearm Discussion forums, part of the Related Topics category; Just interested in opinion here. IIRC something around 18 is a sorta ball park twist rate for many handguns. I realize too that with relatively ...

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Thread: Bullet ''spin''

  1. #1
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    Bullet ''spin''

    Just interested in opinion here.

    IIRC something around 18 is a sorta ball park twist rate for many handguns. I realize too that with relatively short tubes we cannot expect to impart too much spin too soon in a short distance.

    I wonder tho if greater spin was feasable - would there be any benefits regarding terminal ballistics effect - better expansion? Better penetration?

    If I recall right - Muzzle loaders go the other way - something like 48:1 sounds familiar, with MV's around 1200 perhaps - and yet we have something as tight as 7:1 in some .223's - where 3,400 fps is the speed zone.

    Just mulling this over. Interested to hear thoughts.
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    I may be wrong but it seems the smaller the bullet the higher the twist required. It could be because of the smaller bearing surface available to impart spin. Maybe some of the more learned can expound.

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    Senior Member Array WJP9's Avatar
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    Not sure if things brings us slightly off topic....but I believe Speer now offers a "short barrel" cartridge in the GOLD DOT variety that has the benefit of achieving higher MV's. Anybody used these or heard anything about them?
    -Bill

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    I am not a ballistics expert, although I'd like to go to school for it. These are my humble opinions. My experience includes being a gun nut and hunter since 7 years old. I am 25 now.

    The rate of twist differs with the length of the bullet. Many cite the weight of the bullet as being the deciding factor, and in a sense it is, as a longer bullet usually weighs more than a shorter bullet. Longer VLD bullets require a faster rate of twist for the same bullet weight because of the length. That is to say a 65gr Sp will be ok with one twist rate, but a 65gr VLD bullet will keyhole with that same twist rate.

    I think the reason of twist is to stabilize a bullet in flight on a gyroscopic axis. I don't think it has much to do with terminal ballistics. Some of it has to do with internal ballistics, but the main reason for twist is to stabilize a bullet on its gyroscopic axis, thus promoting accuracy and trying to guarentee the bullet enter the target tip first like it was designed.

    The more twist you impart on a bullet, the more friction there is and there will be less velocity, as some of the speed is shrugged off in the name of friction. Take two AR-15s with same barrel length and same everything else (chrome lined, SS or chrome moly bbl) except twist rate. test some loads out of them. There will be a difference.

    Also, the rate of twist used by the makers is to be able to handle ALL bullet weights. So a 16" twist in a .45ACP should be able to stabilize bullets from 165gr to 300gr. This can be seen more apparently in rifles. A .308Win has a 10" twist, nominally speaking. This is designed for 100gr to 220gr bullets. If you are building a target barrel and only shooting one weight bullet, you will need the twist rate matched to this bullet. So a target barrel might have a 8" twist for the heavy 200gr 30cal VLD match bullet where as a 10" twist rate may not stabilize the bullet enough.

    As far as penetration and expansion goes, those vehicles are driven by bullet design and velocity, mainly. A bullet is designed to be driven at a certain velocity for a given cartridge, penetrate a certain target distance, and expand a certain diameter upon aforemention penetration depth.

    External ballistics play some part in this, as the longer the bullet is stabilized on its gyroscopic axis, the more velocity it retains, the more energy it delivers downrange. I suppose you could reduce the twist rate to reduce friction thus increasing velocity, but you might have to change bullet weights depending on your choice of twist.

    There is a stability factor the ammo makers use, 1.5-2.0 being the optimum for accuracy. Where the stability factor of a given bullet is unknown the Greenhill formula can be used. This is good because I don't know how to calculate the stability factor of a bullet.

    The length of the bullet in calibers, multiplied by the twist rate in calibers per turn is equal to 150.

    l= length of bullet in calibers
    t= twist rate in calibers

    l x t=150

    A 173gr 30cal bullet is 4.21 calibers long.

    4.21 x t= 150
    t= 150/4.21
    t=35.62

    Then convert 35.62 calibers into inches, multiply by the bullet diameter (.308) yielding a twist of 10.97096" or approximately 11". So for the 173gr 30 caliber bullet, the optimum twist rate for gyroscopic stability is a 1:11" twist.

    Get a bullet and measure the length. Convert to calibers and plug it in the above formula to find the optimum twist rate. I used to have a bunch of bullets measurements but can't find the list. Just the example of the formula I have on word file.

    Also, if you impart too much spin on a bullet, it will shred its jacket in flight and be inaccurate.

    I hope I answered some of your questions, or at least sparked some more thoughts.
    07/02 FFL/SOT since 2006

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    Thats a good post freakshow.

    The bearing area and weight of the bullet are a factor in determining twist, as well as projected speed of the bullet expressed in feet per second.

    One that comes to mind is one that I am familiar with, the .300 Whisper. It takes a 1 in 8 twist to properly stablize a bullet that is traveling subsonic. Going any more than that and the bullet will start swapping ends in as little as 25 yards.

    That same 1in 8 twist also does pretty good for any weight bullet that travels out of the barrel supersonic. My barrel shoots the 220 grainer at 1000FPS into a less than 1 inch group at 100 yards. It also does quite well shooting 150 grainers around 2200 FPS.

    I beleive that there is a fair range of rifling twists for different calibers, meaning that one could go from 1 in8 to 1in12 for most 30 calibers.

    Although there is an optimum twist for a specific caliber, it would only be optimal for a specific bullet weight with a specific bearing area, meaning the what is the best for one particular bullet may not be the best for a different bullet of the same wieght. Unless one is into competition shooting and shoots only one style and one weight bullet, a "general" twist would probably be more versatile for the average guy. An example would be the guy that would plink with 110 grainers in his .30-06 in the summer time, and deer or bear hunt with the same rifle using a heavier bullet during hunting season.In etheir case, the accuracy is acceptable for both purposes.

    I beleive that you can "overstabilize" a bullet. In this case the bullet spins way faster than it needs to. There is so much centrifigal force on the bullet that it becomes very fragile, literally blowing apart on anything that it hits. This can be a good thing or a bad thing and one that is seen in the .223 caliber. Its a fast mover, but on deer sized game it can blow apart and create a superficial wound that looks horrendous, but actually penetrates very little, creating a nasty wound. The same round on a prarie dog or an armadillo looks spectacular and literally blows them to pieces. In the case of a tougher bullet, such as a military FMJ, the spinning bullet has been known to bounce all over the place and do a bunch more damage in the human body than it ought.

    The twist rate is just one more variable in the whole equation of shooting and it can make or break how a bullet performs. I think that the manufactures do a pretty fair job of determining what is needed and they usaully seem to get it right.
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    I wonder tho if greater spin was feasable - would there be any benefits regarding terminal ballistics effect - better expansion? Better penetration?
    All in all, I think the less spin you have on a bullet the better it will perform in the way of penetration. Using only what is needed is probably the best approach. The more spin that bullet sees, the harder it will be for that bullet to stay together, ecspecially when it hits something and comes to an abrupt stop.

    Thats the way it seems to me anyway, but I could be all wet.
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    I cant say anymore that what has already been said. Except for the new GoldDot short barrel loads. I read a review in one magazine (dont remember which one, I subscribe to 5) about the GoldDot short barrel, so I picked up the first box I saw in 9mm for my Kahr. Though I couldnt perceive much differance between the regular loads and the short barrel loads, the chron showed that the short barrel loads did boost MV by right around 65-70 FPS.
    Fear No Evil.

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    Wow - excellent contributions - thx Freak and HG.

    As an old engineer this subject has always been of interest and some reading has filled some gaps in the past - but I posted much as anything to get some fresh thinking - and got it

    I had forgotten some of the base parameters but this has very usefully refreshed things. Ballistics is a fascinating subject.

    Thx again.
    Chris - P95
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    Wow, Freakshow......there's science in all this gun stuff?

    I always just figured "shoot'em! shoot'em! shoot'em!", 'cause enough lead will sure give'em a "twist".

    No, seriously, that's some good information, clearly explained.

    Thanks!

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    June - SHOOTING TIMES had an article on bullet spin.

    Didja know that a 1:14" twist gives a bullet 144,000 rotations per minute & a 1:7" twist more than doubles it to 308,523 RPM

    A table saw blade rotates a 3,000 RPM...so them there bullets are really spinning.

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    1:7" twist more than doubles it to 308,523 RPM
    Interesting! if that was a .223 then impact after 100yds, taking 3400 as fps - is approx 450 revolutions (in 88 msec)!!!

    And yet - take a handgun bullet at an easy figure of 1000fps and 1:16 twist - at impact in combat distance of 21' - that has rotated just 16 times approx, in 20 msec.!
    Chris - P95
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    If you say so Chris...& if that's your story & you're sticking to it then it's sure 100% A-OK with me.
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    That's an outstanding post, Freakshow...

    As far as penetration and expansion goes, those vehicles are driven by bullet design and velocity, mainly. A bullet is designed to be driven at a certain velocity for a given cartridge, penetrate a certain target distance, and expand a certain diameter upon aforemention penetration depth.
    I would add weight of the projectile in addition to design and velocity. Weight being the more important of the three, IMO.

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    Another consideration on speed and twist is that non-jacketed bullets can only be pushed so fast down the barrel before they start leading the barrel, losing accuracy and increasing pressures. I think muzzleloaders have to take into account the soft lead bullets, being able to shove them down from the muzzle, and black powder fouling.

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    Thought I read about .223 bullets fragmenting before impact thru some AR's.
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