Okay, I have been thinking about this for some time, and I’d like people who actually know something to help me out.

So please tell me if my concept of a principle is incorrect, and where I’ve gone wrong.

1) Bullet stability depends on RPM, that a faster RPM will tend to stabilize better than a slower RPM. (Of course, if the RPM is too fast for the bullet it will go “poof” and disappear in a cloud of gray mist.)

2) If that’s true, then, assuming that we’re talking about the same bullet, there are two variables that impact RPM: rate of twist and velocity.

3) If that’s true, then a faster twist in a shorter barrel will stabilize a longish bullet at a lower velocity, but a slower twist in a longer barrel will stabilize the same bullet if the velocity is greater.

Thoughts?

You appear to be someone truly interested developing a sophisticated understanding of ballistics.

I recommend two books and some chapters from those books.

1. APPLIED BALLISTICS FOR LONG-RANGE SHOOTING, by Bryan Litz, specifically Chapter 10 - "Bullet Stability".

2. MODERN ADVANCEMENTS IN LONG-RANGE SHOOTING, By Bryan Liz, specifically Chapters 1 thru 5 (How Stability Affects Bullet Flight).

Very easy to read and understand.

the speed a bullet spins isn't the only factor in why it poofs.
forcing the bullet forward in the barrel will damage the jacket by skidding the rifling.
once the damaged jacket is in the open the rpm's will exploit those tears.
either undamaged, a thicker jacket, or add more bearing length to stop the skidding and the bullet would make it to the target.

The unit is generally calculated in RPS (second rather than minute) but you are correct. for a given rate of twist, if the bullet has a faster forward velocity it will have a higher RPS than on fired in a same twist barrel at a lower velocity. As such, a 257 Wby may stabilize a 120 grain bullet in a 1-12" twist barrel that would leave a bullet going sideways at 257 Roberts velocities.

there is actually more to determining the optimum RPS than bullet length, but I haven't quite figured it out yet. Part of the equation involves bullet shape which includes the bullet diameter as well as length. the other sacred rule of bulletry involves sectional density being a key factor in penetration. Actually, SD only indicates the average length of a bullet without regard for it's shape, but that's another story.

quote:

Originally posted by B L O'Connor:
If that’s true, then a faster twist in a shorter barrel will stabilize a longish bullet at a lower velocity, but a slower twist in a longer barrel will stabilize the same bullet if the velocity is greater.

True but a longer barrel may not increase velocity as much as you might think. Also, if you increase the barrel length enough, the bullet will start to slow down before it leaves the barrel.

In short, you are correct that the same twist may stabilize a given bullet at a higher velocity but may not do so at a lower velocity.

However, with two different barrels with the same twist, even if each imparts the same velocity to the bullet, one may stabilize and one may not due to other variable qualities of two given barrels.

Figuring out the spin on a bullet is twist and velocity. Figuring out what spin stabilizes a bullet has a different formula. This is Berger's page to do it online.

http://www.bergerbullets.com/twist-rate-calculator/

Whoa there not so fast ( no pun intended

Increasing bullet linear velocity and bullet stability ? Assuming we are referring to static stability ie. SF

We have to consider though that the stability condition is dependent not only on the SF but also the dynamic stability condition as well as the tractability condition.

So lets look at SF only.

Increasing linear velocity increases RPM but the question is by how much and does it have an effect on stability ie SF ?

in order to make sense of it we have to understand how and why spin is necessary to stabilize the bullet.

Also we have to understand the physics behind spin and stability.

RPM's and RPS's are big numbers but when we derive them down to comparable numbers ( which it should be ) it gives better perspective.

Gyro Stability is attained when the overturning force ( moment) of the bullets is offset by the gyro moment. It is the angular momentum of the bullet that is of importance and if we derive angular velocity down to a workable value of a point on the surface of a spinning object and compare that to the linear velocity of the same spot we see that that angular velocity is actually quite small ! ( less than 10 % of the linear velocity ) Not only that increasing or decreasing linear velocity in the same barrel only gives a small commensurate or decrease in angular velocity.

Considering that Gyro stability is basically a ratio between the overturning moment and the gyro moment increasing or decreasing linear velocity by a 100 fps or 200 fps only has a small effect on angular velocity and very little effect on the Gyro moment.

Static stability is attained once the ratio is equal or greater than one !
A bullet that is statically stable at muzzle velocity will be stable down range and that stability increases because linear velocity degrades art a faster rate and angular velocity.

Get copies of Bryan Litz's books and read - you won't regret it.

Though the OP is substantially on the ball, may I suggest that these matters can be over-intellectualised. As Remington has found in the 244, 280 and 35 Whelen, trying to match the rifling to the perfect bullet can end in tears when the buyer decides he wants to load a heavier one.

Better, I think, to opt for the fastest likely twist, and hope that if the shooter wants to load lighter bullets they will be structurally sound and not mind being over-stabilised.

sambarman338.

You are correct on this one. Choosing a faster rate of twist barrel is the way to go for heavier ( longer) bullets. The gains or losses in stability from altering velocity is to small to make a difference. There is off course a rate of twist limit. I don't believe that angular velocity is the cause of bullet break up in flight.

In GENERAL, for a given bullet length, diameter, and weight, there is a range of spin vs. velocity that will be most stable.
However, every time someone goes to prove this for, say, 223 Rem., they find that damn rifle with a 1:12 twist that does great with 60-70gn bullets and that other damn rifle with a 1:8 twist that loves 40 gn bullets.
So, if you are making thousands of rifles, you can select a twist rate for a given cartridge that will work well for almost all of them, but for an individual, you still have find what YOUR rifle likes.
For handguns, almost all have twists rates that a 2-3 times faster then the so-called "optimum."