Twist Rate, Velocity, Accuracy, relationship?

[film495]

So, basically what I was wondering about is when you fire a rifle - say a 30-06 with a 1:10 twist that means basically the projectile would spin 100 times over 1,000 inches of flight, no? or 100 times every 83.3 feet. Well, that would be just the math if the air and environment had no impact on the projectile. So, it made me wonder if the rifling grooves cut into the bullet by the rifling increase or decrease the spin with the friction of the air going through the grooves?

My guess is you would want it to be ever so slightly to cause drag and slow the spin, neutral would be like throwing a knuckle ball, and too much would act like a propeller which can grab any little drift of breeze and end up wild.

So, if you are shooting at longer range, where is the ideal velocity based on twist rate or is there such a thing? I would think you would want the projectile weight, twist rate, and velocity - all in sync, but I poked around and didn't find any thing on this, but it must have been done endlessly, I'm just not searching for it correctly.

 

[troy fairweather]

The purpose of 5R rifling.

 

[jmr40]

5R rifling reduces the friction between bullet and barrel. Theoretically increasing velocity a tiny bit

Twist rate has to do with stabilizing bullets. Generally speaking longer bullets need to spin faster than shorter bullets to be stable in flight. The 30-06 was designed around heavier, longer bullets so they traditionally have a fast 1:10 twist. When the 308 came out it was designed around a shorter 150 gr bullet so they traditionally use a slower 1:12 twist.

With common 150-180 gr bullets either twist rate works fine. If shooting bullets 200gr or heavier then a 12 twist might be a handicap

The general thinking is that if the twist is too slow the bullet is less stable, and less accurate. But it is harder to spin a bullet too fast. Most people today tend to err on the side of using a faster twist than previously.

As long as the bullet is above the speed of sound it will remain stable and accurate. Once it falls below that speed it is unstable in flight and no longer accurate. How long it remains above the speed of sound is determined by aerodynamics and it's initial muzzle velocity.

For most people that is enough to know. When you start getting into EXTREME long range shooting then some of the finer details start to matter. But someone smarter than I will have to explain it.

 

[Grumulkin]

You're overthinking this.

1. Why should you care that a bullet spins 100 times in 1,000 inches of flight? Besides, it's not that way. Velocity will probably decrease out of proportion to spin rate so the bullet will probably spin more thatn 100 times in 1,000 inches of flight if that matters. Keep it simple, right out of a 1:10 twist barrel, a bullet spins 1 time in 10 inches of flight.

2. The rifling grooves will tend to decrease spin.

3. "Propeller" isn't the first thing that comes to mind when thinking of grooves in a bullet.

 

[Aletheia]

The reason you're finding almost nothing about your question is that there is almost no one who worries about the issue. There are many, many variables that do have an effect on trajectory that need to be addressed before worrying about how a bullet that starts at 250,000 rpm might slow down to 248,927 rpm (or whatever the number is) at 1000 meters. The spin rate of a bullet will not decrease very much at all in the time of flight to the target, and to my knowledge, no one in shooting sports worries about bullet spin rate other than making sure the spin is fast enough to make the bullet stable along its axis. Spin rates necessary to do that vary with the length of the bullet, so longer/heavier bullets in a given caliber may need faster twist rates to keep them from wobbling, but I don't know of anyone who worries about spin rates decreasing from friction with the air as the bullet rotates.

I have seen videos of people shooting (handguns mostly) vertically on a lake and finding the bullet on the ice after it falls back down. The bullets are often on the ice spinning like a top for many seconds after they land. Google it to see if you can find any. Bullets spin VERY fast. Fast bullets spin VERY VERY fast, and I have seen a Swift spin them so fast they don't reach the 100 yard target because they disintegrate from the forces created by the spin rate. We could see with the naked eye a "puff" of stuff appear in the air as the bullets came apart.

If I was you, I'd stop worrying about the issue you describe.

 

[Varminterror]

The angular velocity of the bullet (rate of spin) IS reduced in flight. The rate of decay for spin is related to the same factors as the rate of over-land velocity loss, but additional influences can act on the velocity which cannot act on the angular velocity (gravity, as an example).

5R Rifling was NOT designed to reduce form drag of the rotating surface - it was designed to reduce friction in the bore as well as reduce the deformation caused to the bullet, and increase overall seal between the bullet and barrel.

Many discussions have been had regarding the downrange angular velocity of the bullet, and at a high level, it’s quite possible to promote a relatively valid mathematical model. However, these discussions are largely fruitless, as the influence in practical ballistics is effectively nill.

 

[film495]

well, I was just wondering about this stuff cause I like math and science stuff. never seen anything on how the grooves cut into the bullet impact trajectory - wondering if that is why the different twist rates come into play - or if that is more a matter of centrifugal force or something along those lines. it just came to mind that different depth of rifling and twist, shape etc, would impact the air turbulence in flight, and was looking to see if there were some articles on it to read about, just cause I like that sort of thing.

 

[Varminterror]

Twist rate, muzzle velocity, bullet length, and bullet mass must all be balanced to spin the bullet fast enough to stabilize the bullet (conservation of angular momentum) without spinning it so fast the centrifugal forces exceed the structural tolerance of the bullet materials.

The rifling type has exceptionally little (as in “no”) influence upon the turbulence the bullet experiences in flight. Boundary layer theory promotes a pretty simple qualitative analysis of what’s going on, such the velocity due to rotation has nearly nothing to do with the net influence on the bullet. Relatively, the surface of the bullet is “rolling” is about 300fps whereas the bullet is traveling transversely at over 10x that speed. So the boundary layer is shed a little faster than it would if the bullet were not rotating, but overall, it’s not the dominating influence.

What we can observe, in the context you are referencing, is the influence of spin rate on ballistic coefficient. The Litz volume of work, as well as the Berger stability calculator, are informational of this relationship. A bullet’s ballistic coefficient is not maximized when it is only marginally spin stabilized - stability is more of I/O function than a spectrum, such a bullet is either stabilized or it is not, but we note these “marginally stabilized” bullets will have a lower ballistic coefficient than a fully stabilized bullet. Unlike an unstable bullet, this decay of ballistic coefficient isn’t (in theory) caused by inaxiality, eccentricity, or yaw, but rather by an insufficiently developed boundary layer profile.

There is far more at work than a simple bluff body moving through air - we’re balancing a center of mass and center of pressure in a a system with forces acting and inertia’s present in multiple directions.

Consider that a way of saying, “you’re under-knowing your overthinking.” Read Litz’s volume of work, you’ll be far better equipped then to draw more appropriate questions.

 

[film495]

Thanks for pointing me at some references, I was just curious about this and wanted to take in some info on it, and wasn't finding it on my own, didn't know where to look or names or keywords to search on - so, this should get me some thing to read when I can't sleep … lol

 

[Varminterror]

As an engineer myself, I find the physics of all of it enthralling - BUT... most guys who are shooting long range, even extreme long range, have no need for this type of advanced science and mathematics in their craft. Carpenters don’t have to understand a mathematical simulation of the moment arm which is their hammer to be able to drive a nail. I have shelves of books and many GB’s of downloads on hand describing these systems, but at the end of the day, none of them have made me a better shooter. Shooting technique and wind reading are viable knowledge bases for increasing shooting skill - knowledge which begets skill through practice. But such advanced ballistics information - eh, it might increase an understanding, but it doesn’t increase the skill.

In other words - if you want to understand external ballistics, study external ballistics. If you want to be able to shoot long range, study techniques and practice shooting long range.

 

[old heeler]

The angular velocity of the bullet (rate of spin) IS reduced in flight. The rate of decay for spin is related to the same factors as the rate of over-land velocity loss, but additional influences can act on the velocity which cannot act on the angular velocity (gravity, as an example).

5R Rifling was NOT designed to reduce form drag of the rotating surface - it was designed to reduce friction in the bore as well as reduce the deformation caused to the bullet, and increase overall seal between the bullet and barrel.

Many discussions have been had regarding the downrange angular velocity of the bullet, and at a high level, it’s quite possible to promote a relatively valid mathematical model. However, these discussions are largely fruitless, as the influence in practical ballistics is effectively nill.

All you have to do is read Boot Obermeyer on his 5r barrel.

http://obermeyerbarrels.com/faq.html