Re: The theory behind stabilizing bullets.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Mordamer</div><div class="ubbcode-body">What makes bullets unstable?
What can a reloader do to make a certain bullet stabilize better?</div></div>
A. What makes bullets unstable?
1) Insufficient rate of spin
2) Flaws in the bullet resulting in a static imbalance or asymmetric drag
3) Flaws in the barrel's bore, particularly at the muzzle's crown
4) Crossing the transonic region before striking the target (generally but not always the case)
5) Impact with anything of considerable mass
B.What can a reloader do to make a certain bullet stabilize better?
You're asking the wrong question. You select the bullet based on whether your barrel will stabilize it at the anticipated velocity and under the expected atmospheric conditions.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bowslngr</div><div class="ubbcode-body">...Simple answer: any bullet's aerodynamic center of pressure is forward of its center of gravity (mass). Therefore, without being properly spin stabilized, any slight disruption of the bullet's orientation (nose up, down, left, right) can cause it it to tumble....</div></div>
First, <span style="text-decoration: underline">all</span> bullets experience "disruption" at the instant they exit the muzzle. Not only does rotational stability <span style="text-decoration: underline">not</span> correct that condition, it causes the bullet to resist the forces that <span style="text-decoration: underline">would</span> correct it.
If/when that correction occurs, once the spin axis has returned to coincidental with the path of flight, the bullet is said to have "gone to sleep." This obviously is something you want to occur as soon after launch as possible.
It's not uncommon for F-Class benchrest shooters to have loads that will "keyhole" at 100 yards but cut perfectly circular holes at 200 yards and beyond, indicating that their bullets have not yet gone to sleep at the shorter distance.
Second, CP is not a static property, it's dynamic. It moves aft progressively with velocity but takes a pronounced shift aft at or beyond the speed of sound due to the influence of the shock wave. Most spitzer-style bullets require the extra push from the shock wave to produce the positive static longitudinal stability that comes from having the CP aft of the CG. This is why the range at which a centerfire rifle's bullet slows to SoS (or better still, "break" velocity) usually is the distance established as its maximum range. Below SoS, CP shifts forward, which dramatically reduces the margin of stability. Spitzer bullets in general require dramatically higher RPMs to stabilize once they've lost SoS.
This also is why wasp-waited "diabolo" airgun pellets -- which are designed purely for subsonic flight -- are made with their CG so near the meplat and with a large, high-drag skirt at the rear: that configuration guarantees the CP will remain aft of CG, even at subsonic velocities.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bowslngr</div><div class="ubbcode-body">...While you are increasing rotational velocity, you are also increasing the magnitude of the destabilizing force that you are trying to overcome in the first place (increased muzzle velocity means increased aerodynamic forces on the bullet)...</div></div>
You're assuming that an increase in aerodynamic forces will reduce stability when in fact the opposite is true.
The idealized perfectly stable bullet has a spin axis that is tangential to the path of flight. This is the lowest drag condition any bullet can experience, which in turn produces the most predictable path of flight. But that's <span style="text-decoration: underline">never</span> the way a bullet starts out.
Because 1) even slow bullets revolve at a rate of several tens of thousands of RPMs, and 2) no bullet and bore are ever perfect, and 3) no muzzle ever is perfectly motionless at the instant of bullet exit, the bullet <span style="text-decoration: underline">always</span> gets knocked sideways a bit at muzzle exit. It's known as "lateral jump" or "lateral throw-off."
At that point, the differing gyroscopic forces act together to produce a motion known as
nutation, also called "coning." In plain english, that means the bullet is wobbling like a
Billy Kilmer pass. The primary force acting to reduce the coning motion and to cause the bullet to "go to sleep" is aerodynamic drag.
The higher the velocity, the greater the drag. The greater the drag, the further aft the CP is manifested. The further aft the CP is manifested, the more stable the bullet becomes and the sooner all coning is damped out.
Angular momentum changes in linear fashion, meaning 2x the rotational velocity yields 2x the force. Aerodynamic drag, OTOH, changes with the square of velocity, meaning 2x the velocity produces (2^2=) 4x the drag and 3x velocity creates (3^2=) 9x the drag. So as velocity changes, the influence of aerodynamic drag (both on the CP and the bullet in general) changes at a greater rate than does angular momentum. That means that increasing the velocity of an already-stable bullet can never make it unstable whereas reducing its velocity can cause it to take longer to "go to sleep" and might entirely prevent it ever becoming stable.
It's not a bullet's weight that determines how hard it has to be spun to be stable, it the ratio of length to diameter. So long as you're talking about bullets with the same profile/ogive, heavier usually will mean longer, so most folks think of it in those terms. But a round-nosed bullet generally will need fewer RPMs to be stable than will a high ogive spitzer of the same weight because it's shorter, which makes its length-to-diameter ratio lower. I shoot a 155-gr Laupa in my .308 that has to be spun harder than a 168-gr SMK because it has a very gradual taper (and a high ogive number) and is even longer than a 175-gr SMK! So it's not weight, it's that L-to-D ratio.
All of which goes back to my point that you chose a bullet that will be stable (with a wide margin for +/- error) considering your barrel's twist, its anticipated velocity and the expected atmospheric conditions.
