Guns, Bows, Shooting Sports, and Hunting Rifling Twist VS Rotational Speed

[GHarman]

Heres one that many shooters never consider. You think a VW Diesel has a high RPM. What is the RPM of the Bullet fired from your favorite rifle????? YEP, they got RPM's. Thats why they have rifling, to impart a gyroscopic spin on the projectile to keep it, point on, in flight. Here is a basic example. If your favorite rifle has a 1-12" rate of twist in the barrel. The muzzle velocity is 3000 feet per second, the equation is; 3000fpsX12"=3000rpsX60=180,000RPM's! If it has a 1-10" twist the calculation is; 3000fpsX12/10X60=216,000RPM's. Say you have an AR-match rifle with a 1-8" twist and you are using the 80gr. Sierria Match Kings for 1000 yard Competition. The Muzzle velocity is a conservative 2700 FPS. The Calculation is ; 2700X12/8X60=243,000 RPM's. Ya better have good bullets :-laf. GregH

 

[GCroyle]

Keyhole anyone?

Greg,



I just waking up and your time is about 5:13AM and you made my head spin about as fast as a . 223 on its way to the X-ring. I'm still spinning!



Thanks for the head scratcher.

 

[GHarman]

Greg,



I just waking up and your time is about 5:13AM and you made my head spin about as fast as a . 223 on its way to the X-ring. I'm still spinning!



Thanks for the head scratcher.

:-laf Oo. Most folks have not considered this bit of trivia. Yet, Ballistic engineers/manufacturers have to know about this factor to make a quality projectile. The faster you spin a bullet the more the flaws affect its flight characteristics. GregH

 

[WMonroe]

That is something I have never thought about. So do you get better accuracy with more or less bullet rpm?

Will

 

[GCroyle]

I'll try one here, think about a bullet that is not "balanced" and has a slight heavy side, doesn't that make it orbit around an axis as it flies? One time it might be at the low point of the orbit, next time at the high point of the orbit?

 

[fest3er]

The axis will be somewhere within the bullet. Otherwise, I think the bullet's trajectory would be wildly unpredictable.



A slightly unbalanced round should be audible as it travels through the air. I would expect a badly unbalanced round to be clearly audible. I would also expect a badly unbalanced round to 'throw' the barrel around to some degree, spoiling one's aim.



So, by that calculation, my Glock 17 rounds spin about 56,000 RPM. Which may be a little faster than my turbo usually spins. Sorry, I just *had* to relate this thread to diesels.



But it doesn't seem quite right that *that* much energy is converted into rotational energy in such a short time. I could be wrong though.

 

[GHarman]

Simply put, the longer the projectile compared to its diameter, the faster you have to spin it to get stability at standard temperatures and atmospheric density. The Colder it is, adds a greater rotational speed requirement. The most accurate twist for a given bullet is a very specific requirement that you will find in Bench Rest Competition. It is usually the slowest twist that will stabilize the chosen projectile in expected conditions. This exactitude is not practical for a hunting rifle 'cause you may be shooting in summer heat and then hunt Caribou in Alaska in sub Zero Temperatures. SAAMI Specifications prevail however, in over the counter rifles. You can get specific twists only in custom barrels. A projectile can be quite out of balance and fly erratically and you would not know it untill you see the hit on paper. You have the mechanical axis and the axis that is the center of mass. Ideally, in high quality Match Grade Bullets and some of the higher priced Hunting projectiles you will have this greater degree of dependable precision, where both axises are superimposed and parallel. Poor quality bullets may have a microscopic flaw between the core and the jacket that will cause your rifle to shoot a pattern instead of a group. A MG is expected to do just that because of barrel wear and non-Match Grade Ammo. It is called the "Cone of Disbursion". Gary has it close to right as to the flight characteristic of a flawed projectile. Hatchers Notebook has documentation of many external ballistic studies. The RPM's are simple math. 1+1=2; Example. A commercial . 308 Winchester has a barrel with a 1-12" twist. 12"=1' . Muzzle Velocity of a 150 Grain Factory load is ~ 2850 Feet Per Second. That equates to 2850 Revolutions per second times 60 ( number of seconds in a minute) which equals 171,000 Revolutions per minute. Hope this adds clarification.

BTW, Match Grade Projectiles can and often do, exibit YAW after leaving the barrel and it takes a fair distance for them to settle in, commonly called "Go to sleep". Especially long heavy projectiles that have a very pointed nose and a tapered base. GregH

 

[CKelley1]

we built up a 22-6mm wildcat we bought 100o 75 gr hornady bullets, i could not hit the 100 yard target , i moved one to 50 yards no hit so moved to 25 no hit , scratching our heads i fired one while looking over the barrel a grey puff of smoke vapor trail left the barrel , with 1-8 twist at 4,000 fps + it did not stay together more than an inch or two out of the barrel

 

[GHarman]

we built up a 22-6mm wildcat we bought 100o 75 gr hornady bullets, i could not hit the 100 yard target , i moved one to 50 yards no hit so moved to 25 no hit , scratching our heads i fired one while looking over the barrel a grey puff of smoke vapor trail left the barrel , with 1-8 twist at 4,000 fps + it did not stay together more than an inch or two out of the barrel

Have you tried the Sierra 77 grain, 80 Gr. Match Kings or Berger VLD's 70,75,80 Gr. ? You got to have a very tough jacket to withstand 360,000RPM'sOo. GregH

 

[AKaiser]

Simply put, the longer the projectile compared to its diameter, the faster you have to spin it to get stability at standard temperatures and atmospheric density... BTW, Match Grade Projectiles can and often do, exibit YAW after leaving the barrel and it takes a fair distance for them to settle in, commonly called "Go to sleep". Especially long heavy projectiles that have a very pointed nose and a tapered base. GregH

Yes, sectional density. Higher SD bullets like 77 Gr SMK (Sierra Match King) Mk262 and Lapua Scenar 123 grain 6. 5mm wouldn't function very well in a slow twist gun. In fact, the 5. 56mm (. 223Rem, sort of) M855 will not properly stabilize in the M16A1, due to the slower twist rate of 1:12 versus 1:7 twist in an M16A2/3/4/M4. The M855 in an M16A1 is only combat accurate to about 90 meters (about 100yd) due to the slow twist and a high SD bullet. One of the reasons the Mk262 is usable in the M16A2/3/4 is the fact that the barrel twist was set up for the M856 tracer, which is longer than the 62 grain M855 (which is plenty stable in a 1:9 barrel).



Ref: M16 & - Wikipedia, the free encyclopedia (notice all the references in this reference link. )

 

[CKelley1]

the bergers vld is what we went with 1" @ 300 yards

 

[GHarman]

Yes, sectional density. Higher SD bullets like 77 Gr SMK (Sierra Match King) Mk262 and Lapua Scenar 123 grain 6. 5mm wouldn't function very well in a slow twist gun. In fact, the 5. 56mm (. 223Rem, sort of) M855 will not properly stabilize in the M16A1, due to the slower twist rate of 1:12 versus 1:7 twist in an M16A2/3/4/M4. The M855 in an M16A1 is only combat accurate to about 90 meters (about 100yd) due to the slow twist and a high SD bullet. One of the reasons the Mk262 is usable in the M16A2/3/4 is the fact that the barrel twist was set up for the M856 tracer, which is longer than the 62 grain M855 (which is plenty stable in a 1:9 barrel).



Ref: M16 & - Wikipedia, the free encyclopedia (notice all the references in this reference link. )

AKaiser, Right on! The reason for the 1-7" twist is so the tracer would be properly stabilized in sub zero temperatures. GregH

 

[GHarman]

the bergers vld is what we went with 1" @ 300 yards

Excellent Group!!! Excellent Shooting!!! GregH

 

[Paddy McMahon]

I think that those calculations for the rotational speed of the bullet do not compensate for the decay of velocity over the course of the time a bullet is in flight. As a bullet sheds it's initial velocity it will also spin slower as it is obviously covering less distance. This seems to be an exponentially decreasing amount that is proportional to the decrease in velocity occuring during the flight of the bullet. For instance, when the bullet is out at 1000 yards it has lost a significant amount of it's velocity it may very well be spinning at 1 revolution in 12 inches, but it is only travelling half the speed, therefore the RPM calculation taken would be inaccurate. Based on the laws that apply to the conservation of energy I also believe that a bullet will not retain it's inital rotational speed over the time it is in flight. The calculations that are being posted for rotational speed of a bullet are only accurate as taken at the muzzle velocity/muzzle area of the bullets path. I am not trying to show any one up here. I just think this just reaffirms the need for quality balistic components such as bullets, neck sizing, etc.

 

[Dl5treez]

Has anyone else experienced the phenomenon with 5. 56/. 223 rounds of lighter weight (52, 55, etc) bullets climbing and/or erraticly flying in fast twist (1:7) barrels?

 

[GHarman]

I think that those calculations for the rotational speed of the bullet do not compensate for the decay of velocity over the course of the time a bullet is in flight. As a bullet sheds it's initial velocity it will also spin slower as it is obviously covering less distance. This seems to be an exponentially decreasing amount that is proportional to the decrease in velocity occuring during the flight of the bullet. For instance, when the bullet is out at 1000 yards it has lost a significant amount of it's velocity it may very well be spinning at 1 revolution in 12 inches, but it is only travelling half the speed, therefore the RPM calculation taken would be inaccurate. Based on the laws that apply to the conservation of energy I also believe that a bullet will not retain it's inital rotational speed over the time it is in flight. The calculations that are being posted for rotational speed of a bullet are only accurate as taken at the muzzle velocity/muzzle area of the bullets path. I am not trying to show any one up here. I just think this just reaffirms the need for quality balistic components such as bullets, neck sizing, etc.

Paddy McMahon, While I am not a ballistics expert nor a physicist, there have been many discussions on this subject by those that have those qualifications. My reading of this subject has been limited to those published in "Precision Shooting Magazine".

While there is a significant amount of resistance to the forward motion of a projectile in flight, The resistance to rotational speed is very insignificant. Time of flight, even at very long distances is also very short. Rotational decay, however, not actually measured by any method we have today is still within the stabilization parameters for the projectile Sectional density. If it were not, the projectile would loose the ability to fly point forward. Forward velocity decay in free flight is not related to spin RPM. GregH

 

[Paddy McMahon]

Forward velocity loss is related to the spin of a bullet... As the velocity decreases the distance the bullet travels is also decreased, therefore even if the bullet is spinning at the rate of twist in the barrel it takes a longer period of time for it to cover the same amount of distance therefore the "rpm" calculation isn't accurate unless it is observed as an instaneous measurement taken at the muzzle. This is especially true in calibers such as the 220 swift that shed velocity quickly. I think you have a great post here and I agree that the flight time of a bullet especially shot at a shallow trajectory is very, very short. If you were to chronograph a bullet at 1000 yards compared to the muzzle velocity this would illustrate the point I am trying to make here: that the bullet may still be rotating the same, ie: 1 in 12"-0, but that the decrease in velocity over the flight time makes it difficult to truly quantify the revolutions per minute.

 

[fest3er]

Forward velocity loss is related to the spin of a bullet... As the velocity decreases the distance the bullet travels is also decreased, therefore even if the bullet is spinning at the rate of twist in the barrel it takes a longer period of time for it to cover the same amount of distance therefore the "rpm" calculation isn't accurate unless it is observed as an instaneous measurement taken at the muzzle. This is especially true in calibers such as the 220 swift that shed velocity quickly. I think you have a great post here and I agree that the flight time of a bullet especially shot at a shallow trajectory is very, very short. If you were to chronograph a bullet at 1000 yards compared to the muzzle velocity this would illustrate the point I am trying to make here: that the bullet may still be rotating the same, ie: 1 in 12"-0, but that the decrease in velocity over the flight time makes it difficult to truly quantify the revolutions per minute.

Ah. The RPM can be accurately computed as the bullet exits the muzzle. After that, the spin should decrease somewhat due to drag. But the rate of spin is no longer related to the bullet's velocity, because there is no longer any rifling affecting the spin. Consider a bullet that exits a vertical muzzle at 3000 FPS, spinning at 300,000 RPM. When the bullet reaches the acme of the trajectory (where its velocity becomes zero), the bullet will still be spinning rapidly, because gravity affects only the upward velocity, not the spin.



In the rifled barrel, the bullet's spin is determined by the rate of rifling and the bullet's velocity. Let's say the last 5mm of travel in the barrel where the bullet has contact with the rifling and the average velocity of the bullet across those last 5mm determines the bullet's maximum RPM. This will compute to the same RPM as though the bullet had travelled 12" at a constant velocity equal to the average velocity over the last 5mm.



At muzzle exit, the bullet will turn so many times in one foot of travel. As the velocity slows, the bullet will most likely turn more times per foot travelled, because velocity decreases far more quickly than spin. But the RPM doesn't change with forward velocity outside the barrel; it changes only due to drag outside the barrel (or striking a solid object).



Decreasing velocity affects revolutions per unit distance, not revolutions per unit time. RPM at the end of flight should be determinable using aerodynamic drag.



Hmmm. I wonder. Has anyone ever tried to apply threading to a bullet? At such high RPM, I would almost expect that some form of threading on the bullet might help maintain velocity at the expense of spin. Stated differently, the threading could help improve aerodynamics, thus reducing velocity loss due to drag. Putting the equivalent of 'Air Tabs' at the rear of the bullet could reduce the drag effect of vacuum behind the bullet. Finding out what works would be a form of black magic.

 

[GHarman]

Forward velocity loss is related to the spin of a bullet... As the velocity decreases the distance the bullet travels is also decreased, therefore even if the bullet is spinning at the rate of twist in the barrel it takes a longer period of time for it to cover the same amount of distance therefore the "rpm" calculation isn't accurate unless it is observed as an instaneous measurement taken at the muzzle. This is especially true in calibers such as the 220 swift that shed velocity quickly. I think you have a great post here and I agree that the flight time of a bullet especially shot at a shallow trajectory is very, very short. If you were to chronograph a bullet at 1000 yards compared to the muzzle velocity this would illustrate the point I am trying to make here: that the bullet may still be rotating the same, ie: 1 in 12"-0, but that the decrease in velocity over the flight time makes it difficult to truly quantify the revolutions per minute.

Paddy McMahon, Difficult to quantify, definitely! However, Drag effects the forward velocity only. Velocity loss has no effect on the RPM of a bullet in flight (consider a childs TOP, and centripital force)). They are independent variables in free flight. While the RPM's imparted "may slow down" over the long distance, say 1000 yards. The RPM reduction through "rotational drag" (not related to the other drag) is minimal. Remember, that is a different force than the drag in flight. The bullet is effectively reaming a hole in the air that has first been disrupted by a bow wave. The proof is in the bullet hole in the target. I have shot targets at 1000 yards and have also been on "Butt detail", pulling targets for others. My finding is that all the holes were round. (with the exception of bullets fired from a worn out barrel, or a bullet that is to long for the rifling twist, these seldom make it to the target at 1000 yards). The length to diameter ratio of the bullet determines the required spin (Sectional Density). (Ballistic Coefficient, or coefficient of form, along with S. D. determines the remaining velocity down range, along with Relative humidity and atmospheric density) If the RPM decay was significant, many long range shooters would have to find another hobby. The criteria for long range shooting that I keep in mind is a load/bullet combination that remains Supersonic through the target. The cone of dispersion increases in the trans-sonic zone. However, that is a flight charateristic that has nothing to do with the bullet RPM. Holes made in targets by projectiles going subsonic after a supersonic flight are still round.

Another indication that the RPM loss is minimal is comparing bullets that are coated with molydisulfide and a standard uncoated jacketed bullet. At long range, a moly coated bullet will strike several minutes of angle higher on the target than an uncoated bullet, with both started out at the same muzzle velocity. The moly seems to help engrave the bullet without leaving microscopic flash on the bullet surface and makes the bullet slipperier in flight. While that indicates a difference in terminal velocities, there is no evidence of any significant decay in RPM for either example.

Unfortunately, I have all my books packed away, so I can not provide you with specific research material. However, Precision Shooting Magazine has many articles on exterior ballistics and has the Calculus to go with it. You could contact Carroll Plante at Sierra Bullets with questions. I am sure they could provide definitive answers with the credentials to back it up. Good Shootin'. GregH

 

[Paddy McMahon]

OK I give. what I was believing is that the spin of the bullet was always relative to the distance traveled. My means of determining this was simply considering how many rotations a bullet would make utilizing a 1:12 rifling and travelling 1000 fps versus 4000fps-this lead me to belive that the bullet would still be spinning at 1 revolution per foot but the time to travel that one foot would increase as velocity decreased. Not trying to be a troll here, just an avid shooter and hunter. Thanks for getting me straigtened out on this.