Stabilization Mythology
Sit down in a comfy chair, and grab a beverage of choice before you start this one folks, because it's going to be long, and somewhat obscure.
Tonight, I'm back once again to address a common firearms myth; this one actually a bit more technical than most. We're going to talk about bullet stabilization, specifically that of 5.56 nato and .223 bullets.
This is a common subject of misinformation, because most folks don't understand what bullet stabilization is, or how rifling really works for that matter.
Worse, 5.56 is the default chambering of the AR platform rifle, which is the default rifle choice for tactical Tommies everywhere; and as we all know mall ninjas love nothing more than incorrect information.
Actually, to be fair, even otherwise knowledgeable gunnies don't generally understand the variables involved in stabilization, because they don't have a background in the physics or aerodynamics of external ballistics; and because they hear a lot of bull, that sounds kinda OK, and since they don't know any better, take it as truth.
Before we even begin, let me refer you to the best source on ammunition for the AR platform, and 5.56 nato in general; the ammo oracle. There really is no better collection of information on 5.56 ammunition anywhere.
Also, these same principles apply to all elongated bullets (i.e. anything that isn't a ball), no matter what the caliber is; I'm just using the 5.56 as an example, because it is the most common rifle available with many different twist rates, and also the chambering for which the most misinformation is circulating.
Ok, down to business
First, what is stabilization and how does it work?
Modern rifling has a twist, to impart spin to the bullets leaving the barrel. This spin helps to make bullets in flight more stable in two ways:
First, the spin causes gyroscopic stabilization; which is just like how it sounds. A spinning mass resists being disturbed off the axis perpendicular to the direction of rotation due to gyroscopic inertia; which is rigidity in space induced by radially symmetric (which means the forces are the same along all radii - i.e. identical all the way around in all directions) centrifugal forces.
This is the primary component of spin stabilization for pointed bullets (ball bullets are primarily stabilized aerodynamically, because they tumble as well as spin), and it's force component significantly outweighs most of the aerodynamic components of ballistic stability which I will describe in the next section.
In simple English, bullets spin like a top, and they don't "fall over"; just like a top doesn't.
Now, the second element of stabilization caused by rotation, is aerodynamic. Objects rotating in a fluid (and that's what air is), generate radially symmetric lift (again, this means the lift is the same in all directions).
Anyway, this means the bullet is neither pulled down, nor pulled up by these forces (technically this is incorrect in some very small and specific ways, but for purposes of this illustration it is a valid assumption); or rather it is pulled down and pulled up, as well as pulled to all sides and in all radial directions, equally. When something is being pulled in all radial directions equally, just as with a gyroscope, it resists deflection in the axis perpendicular to the pulling forces.
There are two other components of aerodynamic stabilization, and those are form lift (which is the lift created by the shape of the object itself as it passes through the air) and the angles of attack and incidence; but neither are useful in this discussion at the moment.
The first common myth about stabilization, is that the heavier a bullet is, the faster it must spin to be stabilized. In fact this isn't really true, heavier objects gyroscopically stablize at lower rotational velocities than lighter objects (the flywheel effect); and though the aerodynamic stabilization componenst required for heavier objects are greater in magnitude than for lighter objects, the differences in weight and relative difference between force components, between different examples of the same diameter bullet loaded for the same cartridge, are generally small enough that the aerodynamic component of the stabilizing forces required do not change significantly.
The issue with bullet stabilization is actually length not weight; but because the diameter of the bullet is fixed (we are after all talking about different bullets in the same caliber), there are really only three things which generally change the weight:
1. Profile: if the bullet is less tapered, then it will be heavier for a given length, but generally less ballistically efficient (though not always).
2. Construction: If the bullet is solid copper it will weigh less for a given length than a jacketed lead (the 37gr copper solid varmint bullets for example, are the same length as 45gr jacketed lead bullets). If the bullet is a tracer, or steel penetrator type armor piercing it will also weigh less than a solid lead jacketed bullet for a given length. Also if a bullet is hollow (or partially hollow such as some MilSurp .303 or 7.62r loads), it will obviously weigh less for a given length.
3. Length: The longer the bullet, presuming construction and profile remain the same, the heavier it will be
Generally, this means that a change in length is the same as a change in weight; and since bullet length isn't commonly discussed or published; and because weight is a more important component in interior ballistics, we mostly refer to different bullets and loadings by their weight.
The reason why length is important, is because of the center of gravity, and center of pressure of the bullet; and their relationship.
Center of gravity is a commonly known (if not necessarily commonly understood) concept; which simplified, is the balance point of the bullet. If you very carefully put the bullet on a razors edge at the exact center of gravity, it would in theory balance and just sit there, stable.
The center of pressure is a similar concept; in that it is the point where the aerodynamic forces acting on the bullet are balanced along the longitudinal axis of the bullet (the length).
If your center of gravity, and center of pressure are identical, then your bullet will exhibit exactly neutral stability. This means that the bullet will neither resist deflection, nor will it correct or accelerate any deflection that occurs. Again, if you balance the bullet on the razors edge, it should in theory stay in the same spot until it's disturbed by outside forces.
Of course the center of pressure on a bullet is rarely exactly at the center of gravity. Not only that, but as a bullet accelerates, decelerates, and changes it's angles of attack and incidence (the angle between the longitudinal axis, and the direction of travel; and the relative angle between that axis, and a plane perpendicular to gravity), the center of pressure will change; sometimes radically. Changes in pressure and balance, cause instability.
Now, there is a component of the forces on a bullet called the moment of inertia. The further the action of a force is away from the center of gravity of an object, the greater its moment of inertia. A given force will have a greater effect on an object, at a greater moment of inertia.
In simplified terms, the further away from the CG the force is, the more leverage it has. The more leverage a given force has, the more change it will induce.
How does this apply to bullets in flight?
Well, the longer the bullet is, the greater the moment of the forces, therefore the larger the effect of aerodynamic forces on the bullet are; and more specifically the more they change as the rotational and linear velocity of the bullet change. Additionally, longer bullets have more chances for imperfections, and imperfections also cause changes in the effect of aerodynamic forces.
The very definition of stability is resistance to change; and the more change there is, the less stability there is.
OK, so that what stabilization is, and how it works, but why is it important?
Simply put, stable bullets are predictable.
If a bullet is stable in its flight, it is more likely to hit the same spot as the last bullet.
That is precision.
If a bullet is stable in flight, it is more likely to hit what they are aimed at.
That is accuracy.
Precision produces grouping; accuracy produces scoring (or stopping); both of which are kind of important in the application of firearms for both competitive and practical purposes.
Encapsulated: stable bullets are both more accurate and more precise. Longer bullets require more stabilizing forces to maintain stability. Faster twist means greater gyroscopic effect, and greater stabilizing aerodynamic forces.
Now, the second major myth is "Overstabilization".
Some folks believe that you can "overstabilize" a bullet, and therefore reduce accuracy. For all practical purposes, there is no such thing as overstabilization, so generally faster twist doesn’t hurt accuracy with lighter bullets...
Except in reality it does, for three reasons: unbalanced aerodynamic effects, out of balance bullets, and structural failure due to overspin.
The problem with very light bullets, its that they are more lightly constructed. This makes getting them perfect and consistent and perfectly balanced more difficult. Additionally, any imperfections there are, will have a greater effect because they do not have the mass (and thus the inertia) to resist destabilization.
The faster bullets spin, the more aerodynamic lift is generated. Though the lift is radially symmetrical, thus it balances itself out as explained above; the totality of force is still greater, and therefore there is greater potential energy in case of upset. Very slight imperfections in the bullet cause aerodynamic disturbances which upset and partially offset the stabilizing effects of faster spin.
Those same imperfections, along with slight variations in the distribution of mass throughout the bullet, also cause out of balance “wobble” or precessional destabilization (as opposed to precessional drift, which is motion in the axis and direction of rotation due to gyroscopic forces).
All of these factors upset the bullets to varying degrees, causing instability, and reducing accuracy and precision.
This is true of heavier bullets as well; but their greater inertia makes the forces required to cause upset far greater; as well as increasing the tendency to damp out upsets and return to stable orientation.
Finally, because lighter bullets are more lightly constructed, they are also not as strong; and the faster rotation of higher twist rates causes the internal stresses on the bullet to be higher (torque, centripetal and centrifugal force, shear forces between the layers of construction etc...); which may cause the bullets to disintegrate either in the air from imperfections in the bullet, or on impact; without penetrating the target.
This is common with the lightest varmint bullets driven at very high velocities, and is sometimes called "poofing", or "going poof", because when the bullet disintegrates there is sometimes a visible puff of lead and copper residue in the air. This tendency is unsurprising when you consider that a bullet traveling at 4000 feet per second, may be rotating as fast as 400,000 RPM.
Earlier I said that for all practical purposes, overstabilization doesn't exist in the real world of shooting. This isn't to say that too much gyroscopic stabilization can't occur, but that it's effects are generally so minimal as to be insignificant.
Now, some folks will tell you that you can stabilize a bullet so much, that the bullet wont follow a proper ballistic arc; the gyroscopic effect causing the bullets nose to always point upward at the original angle the bullet was fired at, and cause the bullet to keyhole the target.
To a slight degree this can be true; but only at extremely long or extremely short ranges does this become an issue.
Even some otherwise informed and reputable sources (including the ammo oracle) will tell you this is a problem but this is simply not true of almost all bullets, fired at almost all angles. You would need to have a very short, very heavy bullet, fired at an extreme angle, with a very high twist rate, and with a shape that puts the center of pressure in an odd relationship to the center of gravity, for this to significantly reduce observed accuracy at anything but the absolute shortest, or longest ranges possible.
A bullet in flight will naturally tend to stabilize in a ballistic arc, with the base of the bullet behind the nose, because as the angle of attack changes, the center of pressure will move slightly behind the center of gravity. The base of the bullet acts like the trim tab of an airplanes tail; it tends to react against deflection, and oscilate in a cycle of reducing magnitude until the various forces on the bullet balance out, and stable state is regained.
In general when looking at bullet flight, gravitational forces, thrust forces, gyroscopic forces, and aerodynamic forces will naturally find a balance of ever decreasing magnitude, to produce a smoothe arc. Gravity itself will tend to pull the nose of the bullet down, while deflection lift (equivalent flat plate effect, also called weathervaning) will tend to keep the base of the bullet up. This aerodynamic tendency should overcome any tendency to maintain the initial angle of incidence because of high gyroscopic forces. In a vacuum, that same smoothe arc exists, but the orientation of the bullet will not be stabilized aerodynamically; and the bullet will tend to remain oriented in the intial direction of firing.
The only cases in which this would not be true, is if the range were so short that the bullet had not had time to straighten out from an initial disturbance, or if the range were so long and velocity so low that combined with a very high angle of incidence, a very high angle of attack, and a very high twist rate, the aerodynamic forces on the bullet had reduced to the point where the bullets rigidity in space was stronger than the weathervane effect.
There is one particular component of force that can slightly reduce accuracy due to overstabilization; and that is through precessional drift as described above.
Precessional drift is when a rotating object tends to translate horizontally in the direction of rotation, due to gyrosopic momentum overcoming static inertia (the centrigual force of the flywheel effect pulls the flywheel sideways). If a very light bullet is spun significantly faster than required for stability, at long ranges this precessional drift can slightly reduce accuracy. The heavier, and the longer, a bullet is; the more it will resist this tendency.
Please forgive me if this post was too long, I apologize. I was afraid that most would not read it if it was a link.
Ralf