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Thread: Bullet spin

  1. #1
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    Bullet spin

    Hello all;
    I am trying to figure out how to explain my question, please bear with me. Something I can not get my head wrapped around for some reason is bullet spin rpm.

    I hear and read some rpms in the area of 243,000 or so. (as an example). I have not set out and done the math, just thought experiments. If a given barrel has a 1 in 9 twist, the bullet must spin up after leaving the barrel. Otherwise, it would spin at 9 revs over a foot of distance. The distance could be point blank or 1000 yards. How is it really measured?

    Also, that distance, (point blank or a 1000 yards) goes by (in time) very quickly. I know I have not completely spelled out my thought process here. However, anyone that can continue it and/or throw out some knowledge is greatly appreciated.

    another thought, it would seem at those revs the projectile would break apart from centrifugal force.

    Thanks,
    George

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    1:9 refers to a bullet making 1 complete revolution in every 9 inches of barrel - that's the rate-of-twist.

    So for every foot of travel, the bullet makes 1.333 revolutions (12 inches being 1.333 times the original 9 inches).

    The bullet starts its rotation while in the barrel, of course, due to the rifling imposing spin - not after it has left.

    I'm not quite sure what your question is, though - and yes, bullets can be overstabilized, leading to a bullet tearing itself apart.
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  3. #3
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    No. The bullet doesn't increase its rpms. That's set by the rate of twist in the barrel.

    The rate of twist determines how may times the bullet turns per unit of distance. If the rate is 1:12, it makes one complete revolution in a foot. In 300 yards, it makes 900 revolutions...but the bullet doesn't stay in flight for a minute over a 300-yard distance.

    IIRC...A 168-grain .308 bullet starting with a muzzle velocity of 2600 fps takes about 6 seconds to travel 1,000 yards. With a 1:12 twist rate, the bullet would make 3,000 revolutions in 6 seconds. In a full minute, it would make roughly 30,000 revolutions. So, the bullet in question would be spinning at 30,000 rpms.
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    Well, using the given 1:9 twist = 1.333 rev per foot:

    At 1000 fps (handgun) then that equals 1333 rev per second x 60 = 79,980 rpm.

    Obviously the bullet will have hit something long before a minute has passed.
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    Yes, And my apolgies for the 9" - 1'. Correct, on starting the spin of course after it engages the rifling. I guess what I can not see is how bullets get up to these high rpms from that twist rate. I have not seen it mentioned or have read about it in detail. I think (quite possible mistaken) it was Jim Scoutten that mentioned spins around 240,000 rpms.

    I also have a rule never to post anything while having a drink. Although not drinking, I did take some pain meds for my back before the post...............

    George

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    I guess what I can not see is how bullets get up to these high rpms from that twist rate.
    Ever felt the counter-torque from firing a .357 or .44 magnum ???
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    OK. Actually simple. I think I am just overthinking a simple concept. Thank y'all. I should have done the math.

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    Well, if the bullet was stationary, the RPMs would be impressive to behold, yup - but a bullet moves a heck of a distance in a second, let alone a full minute.

    Same reason people thought the Black Talon was a buzz-saw - forgetting that the average chest might be, let's say, 18" thick - meaning the bullet would only actually rotate twice before exiting.
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    Bullet RPM Formula
    Here is a simple formula for calculating bullet RPM:

    MV x (12/twist rate in inches) x 60 = Bullet RPM

    Quick Version: MV X 720/Twist Rate = RPM

    Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

    Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM.
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    Here are two easy formula:

    MV x (12/twist rate in inches) x 60 = Bullet RPM

    Quick Version: MV X 720/Twist Rate = RPM
    Last edited by Knucklehead2; November 12th, 2008 at 06:47 PM. Reason: I type too slow

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    A lot of the energy in the powder goes to getting the bullet spun up to those high spins. (I'll look up the percentage when I get home.)

    And yes, a quarter of a million RPM is not out of the question --in fact is fairly common.

    To get your mind wrapped around it, look at it this way:

    Suppose a rifle with a 1 in 12 inch twist fires a bullet at 1,000 feet per second.

    That means that the bullet turns around once in one foot.

    That also means the velocity is 1,000 feet per second x 60 = 60,000 feet in one minute. (Yeah, it slows down, but we're just looking at the muzzle velocity here.)

    That means the bullet is spinning 60,000 times in one minute, or 60,000 revolutions per minute, or 60,000 RPM.

    A bullet shot out of this same barrel at 3,000 feet per second will be spinning 3 times as fast, or 180,000 rpm.

    A bullet shot out of that barrel at 3,000 feet per second with a 1 in 9 twist will be spinning at 12/9 X 180,000 = 240,000 rpm

    A bullet shot out of this barrel with that same twist at 3,300 feet per second will be rotating at 3,300/3,000 x 240,000 = 264,000 RPM.

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    muzzle velocity of 2600 fps takes about 6 seconds to travel 1,000 yards
    WHAT?

    Some one help me with this math. Since 1K yards = 3000 feet, and it travels the first 2600 feet in 1 second and the remaining 400 feet in 5 seconds ( I know that is not true, but)? You have got to remove the slowing of the bullet from the equation. I think that the resistance to air of the spin is a lot less than that to traveling distance. Therefore the spin rate will drop at a slower rate than the forward motion.

    If you want an accurate calculation, you must use the measurement at the muzzle and forget about long distance, unless you want to see how much the spin dissipates over distance, and that would be a challenge for even the math whizzes to come up with. Considering the wind resistance of the imparted rifling marks, lack of concentrically of the bullet mass, etc.
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    If you want an accurate calculation
    Wasn't shootin' for an accurate calculation. Just a close approximation to show the OP that 243,000 rpms was...a bit off the scale.
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    Just a close approximation to show the OP that 243,000 rpms was...a bit off the scale.
    Not to bust your chops, but is it??

    contenderman and 230RN calculations show it is not.
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    Not to bust your chops, but is it??
    Bullets light enough to be accelerated to 3,000 fps at a > 1:12 twist rate often will disintegrate themselves in short order from the rpms.


    And remember, the .45ACP (from a 1911, tuned or otherwise) is generally thought of as a relatively "slow and heavy" round.
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    2600 fps will travel 1000 yards in just over one second, not six seconds.

    Bullets light enough to be accelerated to 3,000 fps at a > 1:12 twist rate often will disintegrate themselves in short order from the rpms.
    Nonsense! 5.56 travels at about 3300 fps at twist rates FASTER than 1:12. The current milspec is 1:7. That is almost two revolutions for every foot. 230RN got it right.

    Now a 45 grainer might rip itself apart because the bullet is not long enough to be stabilized. Long bullets require a fast twist. Short bullets require a slow twist. That is why pistols generally have a slower twist rate.
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    I have run 45 grain bullets over a chronograph and gotten 4000 feet per second from a .22-250 AI.
    My 'light' bullet barrel is 1 turn in 12 inches, so those bullets are spinning at 4000 turns per second, or 240,000 RPM.

    I have ripped some bullets apart, but thee were 40 grain bullets designed for around 2000 ft/s.
    When loaded up to 4000 ft/s they could not hold together.

    You cannot 'over stabilize' a bullet, but if you spin it faster than needed for stability you will exaggerate any defects in the bullet and can have accuracy problems.
    A slight imbalance that may have a minor effect at a lower RPM can easily become significant at higher RPM.

  18. #18
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    Hmmm, I'm glad that nobody told my varmint rifles that a bullet light enough to be accelerated to 3000 fps in a 1 in 12 twist would likely come apart.... I'm regularly shooting .22 centerfire bullets well over 3000 fps, sometimes over 4200 fps, and they all make it to at least 100 yard in one piece. There are a lot of prairie dogs that would have been a lot happier if those bullets had come undone though.

    Shot a .22/300 Wby once though that could and would leave nothing but a gray streak on the way to the target with just about any bullet. The bullets usually didn't make it to 100 yards either.

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    This discussion about bullet weights and twist rates brings up a question I've had for some time now.
    I own 2 ARs. How do I find out what their twist rates are? Is this something I'm going to have to have a gunsmith determine?
    Thanks.

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    You can measure the twist rate easily.
    Put a patch on a jag and push it it half way up the barrel.
    Put a piece of tape on the rod, and a black mark (use a marker) facing up on the rod.
    Pull the jag back until the mark is up again and apply another piece of tape.
    The distance between the pieces of tape is the length for one turn on the barrel.

    You need an index on the gun to position the pieces of tape, like the rear of the upper, or any other convenient reference point.

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    Bullet spin over time

    I can attest to bullets holding together at high velocities. The Sierra 110 grain .308 HP will hold together past 4200 ft/sec from a 1:10 twist barrel. That's ~300K RPMs. Accurate too. This was shot out of a .308 wildcat, with a lot of powder.

    If you want to see the change in Bullet RPM as it goes down range, take a look at my Modified Point Mass calculator:

    http://www.eskimo.com/~jbm/calculations/mpm/mpm.html

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    "...the bullet must spin up..." A bullet doesn't do that. Once it's out of the barrel, it's got all the spin it's ever going to get.
    "...How do I find out what their twist rates are?..." Look on the maker's site. Or put a cleaning patch on your cleaning rod and put the patch into the chamber until it stops. Then, put a mark on top of the rod near the handle and one on the rod near the chamber. Push the patch through the barrel until the mark near the handle makes one complete turn. Mark near the chamber again. The distance between the two chamber marks is the twist. One turn in whatever number of inches.
    Heavy for calibre bullets tend to stabilize better with a fast twist than lighter bullets. How fast it's spinning doesn't matter.

  23. #23
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    Not to bust your chops
    Well...If it makes ya feel any better, break out the slide rule. I don't have time these days, and any response on the threads generally has to be a quick hit and run.

    And remember, the .45ACP (from a 1911, tuned or otherwise) is generally thought of as a relatively "slow and heavy" round.
    Doesn't matter. If the pitch is 1:12 the bullet will make one turn per 12 inches of forward travel, regardless of how fast it's going.

    I own 2 ARs. How do I find out what their twist rates are? Is this something I'm going to have to have a gunsmith determine? Thanks.
    The trend in recent years has been 1:9 in most rifles. For a while, 1:7 was tried, but they found out that it burned up barrels while-u-wait.
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    [B]What in the World !

    I have read this thread in absolute amazement.. Listen up kids. Twist rate, muzzle velocity, bullet weight etc. have nothing to do with what happens to a bullet after it leaves the end of the gun! Bullet RPM's of 180,000-240,000 is preposterous!

    The millisecond a bullet leaves the gun, it is no longer being acted on by the physics that propelled it out of the barrel. Upon exiting, it is acted on by the two forces of atmospheric (air) resistance, and gravity. Thats all that can happen. The bullet is stuck with it's inertial speed and spin upon leaving the barrel, and neither speed or spin can INCREASE after exiting said barrel. Only the external forces mentioned can act on it now, causing it to slow it's speed and spin.

    The examples I have read are crazy.. If you use the 1:12 twist rate mentioned, then a target 1000 yards away is exactly 3000 feet away. If the bullet turns once in 12 inches (one foot) then it can only physically turn 3000 times before it hits the target 3000 feet away! Now, speed can have it hit the target in 1 second (velocity of 3000 fps) or it could hit the target in 3 seconds (velocity of 1000 fps), in either case, the velocity only effects the time it takes to hit the target. It has nothing to do with spin rate. In fact, air resistance will slow the spin rate, and speed, prior to hitting the target. So the figures mentioned are the maximums attainable, rather than the reality of the bullet having imperceptibly slowed down prior to hitting the target. Think of throwing a curve ball from the mound to home plate, apply the same principles and you will understand the physics easily... be safe...
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    I have read this thread in absolute amazement.. Listen up kids. Twist rate, muzzle velocity, bullet weight etc. have nothing to do with what happens to a bullet after it leaves the end of the gun! Bullet RPM's of 180,000-240,000 is preposterous!
    No it's not. I suspect you have no idea what RPM means. It means Revolutions Per Minute.

    The millisecond a bullet leaves the gun, it is no longer being acted on by the physics that propelled it out of the barrel. Upon exiting, it is acted on by the two forces of atmospheric (air) resistance, and gravity. Thats all that can happen. The bullet is stuck with it's inertial speed and spin upon leaving the
    I think only one person suggested otherwise and that was the initial poster.

    barrel, and neither speed or spin can INCREASE after exiting said barrel. Only the external forces mentioned can act on it now, causing it to slow it's speed and spin.
    Very true.

    The examples I have read are crazy.. If you use the 1:12 twist rate mentioned, then a target 1000 yards away is exactly 3000 feet away. If the bullet turns once in 12 inches (one foot) then it can only physically turn 3000 times before it hits the target 3000 feet away! Now, speed can have it hit the target in 1 second (velocity of 3000 fps) or it could hit the target in 3 seconds (velocity of 1000 fps), in either case, the velocity only effects the time it takes to hit the target. It has nothing to do with spin rate. In fact, air resistance will slow the spin rate, and speed, prior to hitting the target. So the figures mentioned are the maximums attainable, rather than the reality of the bullet having imperceptibly slowed down prior to hitting the target. Think of throwing a curve ball from the mound to home plate, apply the same principles and you will understand the physics easily... be safe...
    Again, I suggest you learn what RPM means. Velocity has everything to do with how fast the bullet spins when it leaves the barrel. Using your example (neglecting air resistance to make the math simple). If the bullet leaves a barrel with a 12" twist at 3000 f/s and hits a target 1000 yards (3000 feet) away, you're right, it would spin 3000 times before it got there. That time of flight would be 1 second. So in one minute, the bullet would have spun 3000*60 or 180000 RPM (again, Revolutions Per Minute).

    If the bullet left the muzzle at 1500 f/s, it would still spin 3000 times on it's way to the target which would take 2 seconds. That means that the bullet would spin 3000*30 times in one minute or 90000 RPM. Note the dependency on velocity.

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