Hello,can somebody give me a quick rundown please on the subject of barrel twist & how it would relate to weight of bullets used,speed & accuracy.Thanks.#1 45-70,Montana 45-90
Barrel Twist
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Bombay club
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Barrel Twist
Hello,can somebody give me a quick rundown please on the subject of barrel twist & how it would relate to weight of bullets used,speed & accuracy.Thanks.#1 45-70,Montana 45-90
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Bumper
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BC, Twist rate in BPCR's is roughly determined by the Greenhill formula. I do not have the formula handy but it takes into account bullet velocity, length, caliber, and weight, giving you an estimate of twist rate needed to stabilize a chosen bullet. Dan Theodore is one of many other knowledgable people on this board can that can better explain how the formula works. Simply put, as a bullet in any caliber has to grow in length to increase weight, the twist rate will have to increase ( from 1:18 to 1:16 for example), so that the longer bullet will stabilize at a given velocity. Rbump-
Bad Ass Wallace
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The Greenhill Formula is often written as:
Minimum Required Twist Rate = (150 * bullet_diameter^2) / bullet_length
Or
Maximum Bullet Length = (150 * bullet_diameter^2) / twist_rate
As you can see there is no accounting for muzzle velocity, spin rate or bullet design. Yes, twist is in the equation, but without velocity, spin rate is not comprehended.
There are better methods to determine the requirements for optimum bullet stability. The downside is that most of the research done by the US Army at the Ballistics Research Lab (BRL) was performed on modern projectiles. But, through testing and making some small adjustments, it is now possible to use the stability equations developed at the BRL to better understand BPCR bullet stability issues focused on silhouette, midrange and longrange match shooting. Those adjustments will be the topic of a later discussion.
But, let us digress for a moment back to the Greenhill Formula. At this time it is suggested that a Greenhill constant of 125 be used for all of the above-mentioned BPCR shooting sports. "150" is the constant used in the original Greenhill formula. Also, for velocities from about 1,000 to 1,400 fps this fudge factor works well. One area where even this "fudge" factor does not work is when the bullet becomes longer than about 1.500" in length. There seems to be some non-linearity in stability when the bullets are a bit longer than 1.500". Also, as the caliber becomes progressively smaller, stability requirements are somewhat non-linear. But, this fudge factor has held reasonably well for bullet lengths between 1.20" and 1.500" using calibers from 32 to 45. So far the following calibers and twists have been tested extensively by this crank, 45-caliber data has been collected from several top shooters who shoot 45-70’s for silhouette and midrange and 45-90’s for longrange:
40 Cal: 16 twist
38 Cal: 10, 12, 14 & 15 twists
35 Cal: 10 & 12 twists
32 Cal: 10 twist (a 9-twist will be tested in the near future)
Multiple bullets in each caliber have been tested for accuracy and yaw angle at ranges from 50 to 1,000 yards. The design issues of nose diameter, ogive radius, bearing surface length and grease groove design have also been tested for their effects on accuracy and stability. All the preceding bullet design parameters effect bullet stability. That is why a bullet of a certain caliber and weight will produce round holes on a 100-yard target and shoot through the wind well and some of the same caliber and similar weight will not.
One way of thinking about bullet stability is to reduce it to the most basic concepts. This BPCR rifle crank is a reductonist, that is he wades through all the "stuff" and then distills it down to the essence of the matter and uses the distillation to move forward with designing rifles, bullets and loads. The heavy lifting is necessary to perform the distillation process properly, but once that is done information is easier to use and many more cranks can use the findings to better design their rifles, bullets and loads.
There are a number of forces acting on a bullet as it flies down-range that tend to cause it to wobble or even tumble. The rotational spin imparted by the barrel twist rate and muzzle velocity act to keep the bullet from wobbling or tumbling. The easiest way to visualize what is happening to a bullet, as it flies downrange with regard to stability, is to imagine all of the aerodynamic forces that tend to make the bullet wobble or tumble as a single force pushing up on the bullet somewhere between the center of bullet mass and the nose of the bullet. This single force is called the center-of-pressure. The center-of-mass is the point near the middle of the bullet that will allow it to balance on the edge of a very sharp blade. Actually it is along the centerline of the major axis of the bullet, but the bullet balance point works for this discussion. Through bullet design we can move the center-of-mass forward and the center-of-pressure (the vector sum of all the aerodynamic forces that tend to overturn the bullet by pushing up on the bullet) rearward. When the distance between the center-of-mass and center-of-pressure is minimized the tendency for the bullet to wobble or overturn is minimized. Think of the distance between center-of-mass and center-of-pressure as a lever arm that is working to make the bullet wobble or tumble. The shorter the lever arm, the less force the center-of-pressure can exert on the bullet to make it wobble or tumble. To further show how this works, a round ball has the center-of-pressure and center-of-mass at the same point. There is no lever-arm to overturn the bullet, so no spin is required to stabilize a round ball.
The stabilizing effects of rotation increase by the square of the spin rate and therefor also by the muzzle velocity increase. The overturning effects of center-of-pressure increase only by the increase in velocity, not the square of the velocity. That is why driving a bullet faster will increase stability. Another way to state this phenomena is that the stabilizing effects of spin rate increase faster than the overturning effects of increased velocity as the bullet is driven faster and faster.
By shooting bullets through corrugated cardboard and measuring yaw angle or tipping, one can start to understand what is required to properly stabilize a cast or swaged BPCR bullet. These field results have shown that a stability factor of about 2.50 or greater is necessary for BPCR bullets so that optimum wind deflection, stability and accuracy are obtained. For highpower jacketed bullets the standard for stability is about 1.50. The 6mm PPC crowd is closer to 1.0 for their standard stability factor. Stability factor is a modern measure of stability used by most exterior ballistics programs. The reason BPCR bullets require much higher stability factors is because our bullets are traveling through the transonic region, about 1,250 down to about 1,000 fps. There is a considerable amount of turbulence in that region so BPCR bullets must be spun much faster than bullets that are launched at high supersonic velocities and stay supersonic all the way to the target. The highpower 1,000-yard shooters are very concerned about making sure their bullets remain supersonic all the way to the target because once their bullets hit the transonic region they begin to yaw excessively and will eventually tumble ruining accuracy.
One of the reasons past work with sub 40-calibers for silhouette were not successful was the use of too slow of a twist for the caliber and bullet length. The physics of exterior ballistics said that the sub-caliber bullets would shoot through the wind as well as the heavy 45-caliber bullets. This was not the result that was observed back then due to the large yaw angles of the sub-caliber bullets, which resulted from too slow of a twist. Now that several have worked out the issue of twist, the sub-calibers are performing well in silhouette and are starting to take their place in longrange shooting also.
"
from BPCR forum by Dan Theodore
Minimum Required Twist Rate = (150 * bullet_diameter^2) / bullet_length
Or
Maximum Bullet Length = (150 * bullet_diameter^2) / twist_rate
As you can see there is no accounting for muzzle velocity, spin rate or bullet design. Yes, twist is in the equation, but without velocity, spin rate is not comprehended.
There are better methods to determine the requirements for optimum bullet stability. The downside is that most of the research done by the US Army at the Ballistics Research Lab (BRL) was performed on modern projectiles. But, through testing and making some small adjustments, it is now possible to use the stability equations developed at the BRL to better understand BPCR bullet stability issues focused on silhouette, midrange and longrange match shooting. Those adjustments will be the topic of a later discussion.
But, let us digress for a moment back to the Greenhill Formula. At this time it is suggested that a Greenhill constant of 125 be used for all of the above-mentioned BPCR shooting sports. "150" is the constant used in the original Greenhill formula. Also, for velocities from about 1,000 to 1,400 fps this fudge factor works well. One area where even this "fudge" factor does not work is when the bullet becomes longer than about 1.500" in length. There seems to be some non-linearity in stability when the bullets are a bit longer than 1.500". Also, as the caliber becomes progressively smaller, stability requirements are somewhat non-linear. But, this fudge factor has held reasonably well for bullet lengths between 1.20" and 1.500" using calibers from 32 to 45. So far the following calibers and twists have been tested extensively by this crank, 45-caliber data has been collected from several top shooters who shoot 45-70’s for silhouette and midrange and 45-90’s for longrange:
40 Cal: 16 twist
38 Cal: 10, 12, 14 & 15 twists
35 Cal: 10 & 12 twists
32 Cal: 10 twist (a 9-twist will be tested in the near future)
Multiple bullets in each caliber have been tested for accuracy and yaw angle at ranges from 50 to 1,000 yards. The design issues of nose diameter, ogive radius, bearing surface length and grease groove design have also been tested for their effects on accuracy and stability. All the preceding bullet design parameters effect bullet stability. That is why a bullet of a certain caliber and weight will produce round holes on a 100-yard target and shoot through the wind well and some of the same caliber and similar weight will not.
One way of thinking about bullet stability is to reduce it to the most basic concepts. This BPCR rifle crank is a reductonist, that is he wades through all the "stuff" and then distills it down to the essence of the matter and uses the distillation to move forward with designing rifles, bullets and loads. The heavy lifting is necessary to perform the distillation process properly, but once that is done information is easier to use and many more cranks can use the findings to better design their rifles, bullets and loads.
There are a number of forces acting on a bullet as it flies down-range that tend to cause it to wobble or even tumble. The rotational spin imparted by the barrel twist rate and muzzle velocity act to keep the bullet from wobbling or tumbling. The easiest way to visualize what is happening to a bullet, as it flies downrange with regard to stability, is to imagine all of the aerodynamic forces that tend to make the bullet wobble or tumble as a single force pushing up on the bullet somewhere between the center of bullet mass and the nose of the bullet. This single force is called the center-of-pressure. The center-of-mass is the point near the middle of the bullet that will allow it to balance on the edge of a very sharp blade. Actually it is along the centerline of the major axis of the bullet, but the bullet balance point works for this discussion. Through bullet design we can move the center-of-mass forward and the center-of-pressure (the vector sum of all the aerodynamic forces that tend to overturn the bullet by pushing up on the bullet) rearward. When the distance between the center-of-mass and center-of-pressure is minimized the tendency for the bullet to wobble or overturn is minimized. Think of the distance between center-of-mass and center-of-pressure as a lever arm that is working to make the bullet wobble or tumble. The shorter the lever arm, the less force the center-of-pressure can exert on the bullet to make it wobble or tumble. To further show how this works, a round ball has the center-of-pressure and center-of-mass at the same point. There is no lever-arm to overturn the bullet, so no spin is required to stabilize a round ball.
The stabilizing effects of rotation increase by the square of the spin rate and therefor also by the muzzle velocity increase. The overturning effects of center-of-pressure increase only by the increase in velocity, not the square of the velocity. That is why driving a bullet faster will increase stability. Another way to state this phenomena is that the stabilizing effects of spin rate increase faster than the overturning effects of increased velocity as the bullet is driven faster and faster.
By shooting bullets through corrugated cardboard and measuring yaw angle or tipping, one can start to understand what is required to properly stabilize a cast or swaged BPCR bullet. These field results have shown that a stability factor of about 2.50 or greater is necessary for BPCR bullets so that optimum wind deflection, stability and accuracy are obtained. For highpower jacketed bullets the standard for stability is about 1.50. The 6mm PPC crowd is closer to 1.0 for their standard stability factor. Stability factor is a modern measure of stability used by most exterior ballistics programs. The reason BPCR bullets require much higher stability factors is because our bullets are traveling through the transonic region, about 1,250 down to about 1,000 fps. There is a considerable amount of turbulence in that region so BPCR bullets must be spun much faster than bullets that are launched at high supersonic velocities and stay supersonic all the way to the target. The highpower 1,000-yard shooters are very concerned about making sure their bullets remain supersonic all the way to the target because once their bullets hit the transonic region they begin to yaw excessively and will eventually tumble ruining accuracy.
One of the reasons past work with sub 40-calibers for silhouette were not successful was the use of too slow of a twist for the caliber and bullet length. The physics of exterior ballistics said that the sub-caliber bullets would shoot through the wind as well as the heavy 45-caliber bullets. This was not the result that was observed back then due to the large yaw angles of the sub-caliber bullets, which resulted from too slow of a twist. Now that several have worked out the issue of twist, the sub-calibers are performing well in silhouette and are starting to take their place in longrange shooting also.
"
from BPCR forum by Dan Theodore
Hold still Varmint, while I plugs yer!
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Bombay club
- Posts: 80
- Joined: Fri Mar 05, 2004 8:10 pm
- Location: grand rapids,mi
barrel twist
thank you both for the valuable information,the more I shoot my shiloh's the more interesting it all becomes!I am curious as to how most shooters arrive at their favorite bullet weight & velocities,is it mostly range work (which is all that I have been doing),or does it involve some paperwork & calculation.
thanks again.
#1 45-70,montana 45-90,waiting on the 45-110!
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Bad Ass Wallace
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- Location: Australia