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[ARCHIVED THREAD] - How fast does a .223 spin? (Page 1 of 2)
Posted: 10/12/2006 3:15:43 PM EDT
| Anyone have any clue how fast a .223 is spinning when it ones out of the barrel? Aproximate RPM? I imagine that it's kind of like doing a burnout down the barrel - picking up a little spin along the way. There was a thread about bullets spinning to pieces out of a 1:7 twist barrel. Could that really happen? |
Not if you have M4 feed ramps. |
Because the RPM of a mass equals energy in addition to its mass x velocity squared. So if in the first 4-8 inches of penetration the bullet is stopped from spinning, where did that energy go? Imagine grabbing a spinning drill chuck at 6,000 RPM. Sort of like this (on the quick)if the bullet is going 3,000 feet per second, and is spinning at a rate like one rpm per half-foot, then that is about 6,000 rpm. When I have done gelatine block shots with the 1:7 twist M16A2, I have noticed radial tearing from the absorbtion of this rpm along the bullet channel when the wound channel is viewed from the entry point. No doubt about it, speed, mass, and rpm kill. |
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Feed Ramps, I think he was pulling your leg. I have seen lightweight varmit bullets spin apart back when the 1/7 AR's were first used in IPSC or 3 Gun matches. You'd see a small puff of grey smoke about 20 yards out. That meant there would be no holes on the target. Good Luck Bill |
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Thursdays=M4 ramp discussion.... Ok, here's an over simplifcation for you rpm question. It's probably not even close to being right. I'm sure someone who knows physics will be along shortly to prove me wrong. The answer is 87 RPM!!! Just kidding, of course. Here's my, for lack of a better term, calculation: Assume average velocity of 3000 fps. 3000 ft = 36,0000 inches. Take your 36,000 inches, the distance traveled in 1 second. Your rifling twist is 1:7, that is to say that the grooves make one full revolution every 7 inches. Theoretically, the grooves engrave the bullet, causing it to spin at the same rate as the rifling twist. Therefore, you take 36,000 and divide it by 7 (both units in inches) and you get 5142.86. The bullet has made this many revolutions in one second. Multiply 5142.86 by 60 (60 seconds in a minute), and you get 308,571.43 rpm. Sounds way off to me. I'm sure there's a formula for this somewhere, and it probably involves converting to radians and using theta to obtain the answer... Somebody help us! |
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Feed ramps have absolutely nothing to do with bullet RPM as it leaves the barrel. twist rate and muzzle velocity. If your bullet is travelling 2000 feet per second out of a gun with a 1 in 7 inch twist it works out as follows: 2000 fps = 24000 inches per second the barrel is turning the bullet at one complete revolution for every seven inches of travel... so you get about 3428 complete revolutions per second. or about 205,714 revolutions per minute (rpm). |
your math works out just fine, 1:7 twist yields ~300K RPM. this angular velocity remains relatively constant during the entire ballistic trip of the bullet, unlike the linear velocity which is decreasing quite quickly. note that there is a popular internet myth that a 5.56mm round "bores" into you like a drill bit might. no so. at a reasonable range, figure penetration to be anywhere from 7" to 14" in human flesh. so the bullet rotates once, maybe twice at best during the path. moreover, the bullet rapidly loses angular and linear velocity once it is upset from the approach trajectory (i.e. while it is yawing). so while the spin contributes some energy to the deformation and subsequent fragmentation of the yawing bullet, it is not a major contributor. hence, once the linear velocity decreases below some critical rate (M193 out of a 14.5" bbl at say 200+ yds), the bullet will no longer fragment and cause a large wound channel -- despite the fact that it is still rotating at ~300K RPM. ar-jedi |
So, the general consensus is that, given a constant velocity of 3,000 fps and a 1:7 barrel, the bullet's rpm will be, roughly 308,000. Many have had experience with thin jacketed bullets vaporizing. Hope this answers the question. 
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First thread I've ever seen that compares bullet spin to wounding capability. Interesting. Is your figure of 6,000 RPM a measured fact or educated guess? Not that it really matters, it's just good AR15 trivia to me. |
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Wait a minute - I just read some of the replys & I'm no physicist, but i guarantee that no bullet in hostory has ever spun at 300,000 revolutions per minute. Wouldn't that open a black hole or something?? Come on. You know when they film bullets going through cards and apples, the bullet has no conceivable spin in a 3-4 inch distance. |
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coldblue - total BS post "...one rpm per half foot..." Does that mean one revolution per minute per half foot? that's almost getting into the inverse of the acceleration rate of african swallows. "...radial tearing from the absorbtion of this rpm along the bullet channel..." So you've been able to measure the creases along the inside of a geletin path caused by bullet rotation? |
I think you guarantee wrong Boombastic. When you are watching those hi res films of bullets how can you perceive the spin rate. At a certain point it becomes inperceivable... kind of like a light bulb - it's actually flashing on and off with the frequency of the AC fed to it, but we don't perceive that. |
The bullet only flies for 1-2 seconds, max. |
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Also, thats rpM, as in Minute. The round is not in the air for close to one entire minute, so, even though that figure sounds high, realise that in the fraction of, or an entire second or so that the round is tearing through the air, only a portion of those 300k rotations are occurring. The math sounds dead-on. The perspective is foggy, because we aren't discussing how long it takes for a round to travel varying distances. I vote for an 'GTG' on the math. 5k rotations in one second. Best, |
I guess that is really my question. Does a 1:7 barrel actually spin the bullet at the same speed, or is there other physical forces that would keep it from doing so. I have to admit though - that even though 300,000 RPM sounds ridiculous to me. It appears to be the correct answer  . See Link |
What's up Marksman, you still allowed to have Ar15's in the Peoples Republic of Illinois?? J/K I moved down here to Texas a year ago from the Schaumburg area. The Anti's finally chased me back down to a more friendly state. Peace. |
Observing identical gel blocks fired with 1:12 twist M16A1 barrels during the same tests, same day,etc., there was no visual indication (as there was with the 1"7's) of radial tearing. All I am saying (call is BS if you like) is what I have seen, measured and observed from straight-up Government tests the Marine Corps paid the Army to conduct. I was there. Where have you been and what have you seen? |
Thank you coldblue - but you still haven't defined rpm per half foot, nor have you defined what you measured exactly. |
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I did this once, just multiply by 1: example 3200 feet/second * 1 rev/7 inches * 12 inch/1 foot * 60 seconds/1 minute = (3200 feet * 1 rev * 12 inches * 60 seconds)/(1 second * 7 inches * 1 foot * 1 minute) = 3200 (3200 * 720 revs)/(7 mins) = 329,142.8571 rpms or (muzzle velocity in fps) * 720/(inches per twist) |
1/7 twist =1.4166 revs. per foot .....is this what you are asking? |
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Because the RPM of a mass equals energy in addition to its mass x velocity squared. Kinetic energy = .5 x mass x (velocity^2) The spinning component (angular momentum) is very small compared to the linear momentum (mass x velocity) So if in the first 4-8 inches of penetration the bullet is stopped from spinning, where did that energy go? Heat generation Imagine grabbing a spinning drill chuck at 6,000 RPM. Sort of like this (on the quick)if the bullet is going 3,000 feet per second, and is spinning at a rate like one rpm per half-foot, then that is about 6,000 rpm. When I have done gelatine block shots with the 1:7 twist M16A2, I have noticed radial tearing from the absorbtion of this rpm along the bullet channel when the wound channel is viewed from the entry point. No doubt about it, speed, mass, and rpm kill. Think of it this way..shooting a bullet straight up in the air and being able to let it drop in your hand after it reaches the apex and begins its descent. Wouldn't break the skin but would probably burn like hell (assuming no loss of rpm from air resistance and room temp metal). I also calculated 308K rpms...fwiw |
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Let do a little more math to figure out about how much energy there is in the spinning bullet. First a little set up. As stated KE = 1/2mv(squared). Lets calculate the velocity of the spin. To do that (and make the math easier than calculas) consider how the bullet can be represented. The most straight forward is half the mass, half the bullet radius, on each side. So you have M--C--M, where M is half the mass, C is the center and the distance from M to C is half the radius. For a .224 bullet half the radius is 0.056. Travelling at 3000fps at 1:7 gives 5100 rps. The distance travelled by the rotating mass is 2*pi*r*5100. Thats about 150fps. So the spin on the bullet is about the same energy as the bullet coming out of the barrel at 150fps. That is a little less than 120mph. IMO not much energy. |
| The rev 300k rpm limit is for bullet shearing is from Sierra Jacketed Match Bullets. Most try to load to around 2800 fps for 1-7.7 twist barrels for match shooting to give a measure of assurance to not overly deform the bullets. Most people that use the 1-6.7 twist barrels to use 90 grainers have a hard time keeping them slow enough as not to shear. I personally have seen plenty of hot surplus miliatary ammo just fly apart not far from the barrel from 1-7 twist barrels. I also personally don't use any barrels faster twist than 1-7.7 Kriegers. |
www.ammo-oracle.com/body.htm#twistduh
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"RPM per half foot" was my rough way the other evening of factoring a one revolution per 7 inches of travel, or roughly every half-foot of travel. Then if the bullet travels 3,000 feet (or 1,000 yards) in about a second, then I multiplied two revolutions per foot x 3,000 feet and came up with 6,000 RPM. Now I know that RPM is revolutions per minute, and my 6,000 RPM results from only considering one second of that minute. But multiplying 6,000 by 60 yields way to high a number. And this rotational energy is not applied to a target over the course of a minute, but in about a second. "what I was measuring" was permanent wound cavity, yawl moment, etc., M193 from a 1:12 vs SS109 (now US M855) from a 1:7, and M193 from a 1:7 in about 1983. Wish I could post the pictures here, but I lack that skill set along with those tuned for higher math functions. The radial tearing was something that was very evident on the 1:7 twist blocks, but was lacking on the 1:12. Since the standard protocol is photographing these blocks from the side, this is not readily observable from the flank |
sounds about right, depends on your velocity. i got 308571 rpm even, but i kept my fractions. |
....... don't you mean Revolutions Per Second? and if the round is rotating at ~6,000 RPS, and traveling at 3,000 fps, that would just lead you back to the 2 revolutions per foot figure. I don't see what you're trying to explain with your funny math. |
All I am trying to point out is that the energy (or perhaps better put "damage to flesh")delivered at the target may be more than "mass x velocity (squared). By that I mean the energy it takes to stop the projectile from spinning on its axis. What ever the RPM or RPS (rotations per second) is, that rotational energy is transferred to the target medium in a fraction of a second. So regardless of how well mathematically stable the bullet may be in "air," the nano second it pierces flesh and organs, it is no longer stable (i.e., pint forward) in a very big way and starts to yawl (as the photos above illustrate--particularly the one showing the M193 in the 1:7. I mean if there ever was a "over stabilized" bullet/twist combo, that would be it. However, its yawl point and subsequent tumbling and break-up, etc. is not unlike the others. Sorry the pictures are not that sharp. We (Marines with help from technical people at the Army's Picatinny Arsenal) took these in like 1983, and what you see above are digital photos of the original faded glossies that I took of those 20+ year old photos about 3 years ago. The testing was done under the then existing NATO protocol at 50 meters. We used 20% gelatine in those days. I used these pictures to help the Marine Corps adopt the 1:7 twist for the M16A2 Service Rifle, which was the program I was assigned to at the time at Picatinny by my boss at Quantico (Gen. Al Gray and Lt.Col. Dick Moressco (sp?)). This test was conducted in response to the nay-sayers that had told the Commandant that the new "quick twist barrels would shoot right through like a pencil and not hurt the target, especially at close range." As the photos illustrate, that is not the case. Because these results made these "armchair" ballistic experts look silly, they then demanded our famous 300 meter watermelon shoot. But that is another story...that also proved them wrong, wrong, wrong. |
OK. So the faster spin rate could cause the bullet to remain stable for a small time period longer once it hits flesh, leading to slightly later bullet break-up and deeper wounds? Like you said, though, the rotational energy is dissipated very quickly, so there isn't a very big effect seen. |
No. What I saw by looking lengthwise on these blocks was "radial tearing" not evident in the 1:12 blocks. What you see in a block after the shot is the air trapped when the wound closes in on itself after the temporary cavity is formed. The radial tears were not apparent in the 1:12 blocks. I do not think it takes longer for the higher rpm bullets to yawl because the transition time from air, to a density many more time dense, is so fast and drastic a change that there literally "are no rules." And then when you factor bones, air spaces like the lungs, belt buckles, magazine pouches, clothing differences, etc., there are even less rules. |
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Ok how would 20% ballistic gel compare then to a real human getting hit in the same spot (center mass) with the bullet first hitting the sternum? Would impact of the sternum upset the bullet more and slow down the rpms quicker therefore creating less breakup or would the sternum deform the round but keep it basically intact? Or would it punch right through and show the same results as the gel? I guess I don't understand how ballstic gel at 20% equates to a real human with a relatively thick sternum at center mass. THEY placed the drawing of the guy with the frown so that it represents a center mass shot but didn't place any bone matter there. It sorta looks like "voodoo" science to me in that it does simulate an impact accurately. I am not saying the round wouldn't sting a bit I have seen it practically applied and know it works I guess I just don't get how the guys in white coats think this is a test of impact on a person and can say this round is better or this twist is better. Then again I am a knuckledragging neanderthal end user and only care if the person stops moving. |
[ARCHIVED THREAD] - How fast does a .223 spin? (Page 1 of 2)
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