Bullet Stabilization

[Llama Bob]

I'm not 100% sure RPM doesn't have an impact on the target. With a safari solid type bullet, I agree with Brian's logic. But what about an expanding bullet, potentially with sharp chunks of jacket sticking out? Having those rotating (at least until friction stops rotation) might have a non-trivial effect.

Does anyone know for expanding bullets in gel whether they stop rotating and maintain their orientation, or keep rotating? I know they will eventually tumble, but I'm curious what happens before that.

 

[Brian Pfleuger]

Whether or not the "rotating razor" effect might be beneficial on target is another matter.

The "force" of the rotation is what I mean to address. As in, the rotation of the bullet "throws" the varmint, as implied by the quoted text. That notion is simply ludicrous.

The bullet rotating and possibly cutting something important that it might have missed might help, or it might miss something it might have cut because it rotated itself out of the way.

The rotating bullet simply does not exert enough force to do anything destructive, either to the target or to itself. If it did, a bullet would expand on it's way to the target, as soon as it existed the muzzle when the rpm (and therefore the force/torque) are at their highest. No amount of RPM (from any reasonable firearm) is enough to distort solid metal in a bullet.

 

[Colorado Redneck]

Brian--the 22-250 I spoke of in a prior post originally had a 14 twist barrel. It liked 40 gr. Sierra BK's best, but I shot maybe 1000 50 gr. V-max at around 3700 fps over the years. (note---prairie dogs are the fodder for this region, so they have less mass and thickness than your ground hogs.) When I started using the new barrel---9 twist--it was dramatic how much more violence was inflicted on the prairie rats. The innards are literally scattered when the bullet makes a center of mass hit. Maybe---just maybe---I am suffering from the "placebo effect." And the rifle now likes 53 gr. V-max so that might be some my supposedly illusionary observations. Whatever it is, I like it!

 

[Brian Pfleuger]

It's not illusionary, brother, it just ain't the RPMs doing it.

Different bullets, different speeds, different different.

I might even suggest that the forces exerted by the increased twist might do something to make the bullet more frangible on impact. Maybe, I don't know. I do know, though, that the Angular Momentum is simply not there for the RPMs to be doing the work.

Heck, even our woodchucks that can go 9lbs+ can be blown to bits by one shot with my .204 and another with the 22-250 looks like you scared it to death, no hole evident. The next pair, just the opposite.

The only way I can see the RPM having an effect is that the bullet fragments may be more likely to travel closer to perpendicular to the bullet path if they're spinning faster. As such, they stay inside the critter longer, rather than blowing out the back, relatively along the original bullet path.

Someone (who's not me) would have to do the math to see how much that force could redirect the bits. Or just shoot some gel with the same bullets at the same speed but different twists and see what angle the fragments travel.

 

[JohnKSa]

I might even suggest that the forces exerted by the increased twist might do something to make the bullet more frangible on impact.

I've wondered about that. If the bullet is spinning so fast that it's on the verge of failure, the stress of impact could be just enough to push it past the threshold. If that were the case, the bullet would almost explode on impact. A sturdier bullet, or one not under so much stress from spin might hold together resulting in a much less spectacular terminal effect.

It would be interesting to be able to do a lot of experimentation with ballistic gel, very high-velocity bullets and high-speed cameras in situations where the spin could be altered.

 

[Unclenick]

It is possible. One of the effects of spinning up a bullet too fast is core stripping. This is where angular acceleration is so great that a non-bonded core will actually slip inside the bullet jacket by the forces extruding it a little forward and narrower. At exit, the core is actually loose inside the bullet and it an the jacket equalize each other's velocity in proportion to their masses, the core picking up a little rotation, but the jacket losing more. Harold Vaughn measured this happening in a 270 Winchester when it was pushed past about 3150 fps with the bullet he had. He did it by measuring the velocity on a chronograph and the rate of spin with a magnetometer reading a tiny disc magnet he's put in the nose of the bullet. The loose jacket probably does open and strip away more easily on impact, so the core effectively becomes a separate projectile and disintegrates to become part of the red mist.

There's a lot of confusion about "overstablization". For practical purposes, there is no such thing at the relatively flat rifle trajectories, and you can prove this by overspinning copper or bronze solids. There is a phenomenon in high firing angle artillery where a bullet will fail to point into the wind fast enough to keep up with the changing angle of the tangent to the trajectory. This is called failing to trace. The projectile tries to maintain its orientation until the air catches it at an extreme enough angle to start it tumbling. Bryan Litz estimated that at flat firing angles below about 20° angle of departure (IIRC) no bullet suffers from this. That is, at lower firing angles they all correct into the wind, albeit more slowly when spinning fast. The bullet just covers more ground for each epicyclic correcting iteration, but the range over which the net correction occurs doesn't change. So, for example, a typical bullet might have to travel 200 yards before 80% of the initial yaw that occurs as it shifts from spinning on the bore axis to spinning on the axis of the yaw of repose. If you spin it faster, it still damps out 80% over 200 yards, but if you count the number of coning cycles of the tip they are fewer and their radius diminishes more per cycle.

There are still some drawbacks to overspinning in addition to core stripping of conventional construction bullets. First, there is an absolute limit that is one turn in some number of calibers (like 20 calibers or so, IIRC). The British, I think it was, did experiments that found they simply couldn't launch bullets with faster spins than that successfully. They would strip in the rifling at practical pressures and just wouldn't go accurately into the night no matter what they did. So this means there is a limit to how long, in calibers, a bullet can be made before it isn't possible to spin it fast enough to stabilize it.

Another problem is that as spin increases, everything affect by increase in gyroscopic stability factor increases. The yaw of repose increases, so spin drift increases and there is an increase in drag associated with that. The amount by which a bullet's POI increases or decreases with wind increases with that as well. And, of course, you have the issue of wobble if there is any imperfection in bullet mass symmetry. It's all well and good to say modern bullets can all handle it, but long range shooters have found some bullets shooting better or more consistently than others even at normal spin rates, and the added dispersion is caused by drift initiation with lobbing the bullet out of the muzzle with a slightly off-bore-axis CG.

The best stability calculator I've seen is the old McGyro calculator that Robert L. McCoy devised. It's main drawback is it takes more comprehensive entry arguments. Geoffrey Kolbe has it on his site.

Sierra's rule of thumb is still pretty good. For "hunting accuracy" and service rifle match shooting, a gyroscopic stability factor of 1.3 to 3.0 is best. For super precision shooting 1.4 to 1.7 is generally satisfactory, with the edge going to the lower end of that range.

 

[Jim Watson]

The Army likes stability 1.5+, no doubt to allow for junky bullets and hot, dirty, worn guns.

Figure the stability factor for a common pistol bullet.
Then wonder why anybody would care about the twist rate.

 

[JohnKSa]

Bryan Litz estimated that at flat firing angles below about 20° angle of departure (IIRC) no bullet suffers from this.

You can poke around and find experts that disagree with him. And, as an earlier quote I provided demonstrates, you can even find people who believe their experiences unequivocally demonstrate overstabilization. That said, even if Litz is right and it only amounts to being a matter of:

...at lower firing angles they all correct into the wind, albeit more slowly when spinning fast.

The longer amount of time spent not aligned/realigning would still equate to an effective reduction in ballistic coefficient as would the other factors mentioned such as:

The yaw of repose increases, so spin drift increases and there is an increase in drag associated with that.

What everyone agrees on is that as long as the bullet is very uniform and not spun so fast that failure results, that the effects of overstabilization are not generally an issue except at long range.

 

[Llama Bob]

The projectile tries to maintain its orientation until the air catches it at an extreme enough angle to start it tumbling. Bryan Litz estimated that at flat firing angles below about 20° angle of departure (IIRC) no bullet suffers from this.

It's really the angle the nose needs to move through, not the angle above horizontal, that matters. The two are sometimes (as in the case of artillery) related, but not always. High angle small arms shots still aren't affected because the angle of travel only changes a few degrees during flight.

It's also worth noting that the highest RPM projectiles commonly seen in the shooting world are in long range competition, where 1 in 7 or 8 twist 6mm barrels shooting projectiles over 3000 ft/s are the norm. Outside of some oddball varmit configurations, you'll never see a higher RPM. And those shooters most definitely care about practical long range accuracy.

 

[Unclenick]

Bob,

The flat fire case is the worst case. That's why it is used. As you elevate up or down a slope you are getting closer to the vertical case where there is no arc in the trajectory.

…you can even find people who believe their experiences unequivocally demonstrate overstabilization.

John,

I would be interested to hear what their experiences were. I, too, used to believe in effects (not gross, but observable) due to "overstabilization", but as I got more immersed in Robert L. McCoys seminal tome, Modern Exterior Ballistics, I realized the only possible outcome of being "overstabilized" to the point of being unable to remain point into the wind, is tumbling. If you see anything less, then there is another explanation for it.

The reason for the above is that gyroscopic bullet stabilization is an equilibrium balance between gyroscopic rigidity and precession. The reason coning motion happens is the time delay involved in the bullet rotating 180° to move overturning inertia in one direction to work against itself in the opposite direction. The delay causes the correction to overshoot. The faster the spin, the less delay and overshoot. When you spin too slowly the delay is so long that the overshoot grows with each coning cycle. When you spin fast enough it shrinks with each coning cycle. When you spin even faster each coning cycle takes longer but it overshoots less, which is why, even though each cycle takes longer, there are far fewer cycles needed to correct into the wind, and thus the total correction time does not take longer in terms of distance downrange to correct, but actually takes a bit less distance.

A bullet cannot go down range constantly pitched up off the trajectory path because the overturning drag then results in it constantly precessing to the side. Typical yaw of repose has, say, a tenth of an moa of pitch up off the tangent to the trajectory path, and five to ten times that in sideslip yaw. This is so the sideslip yaw, at equilibrium, produces enough downward precession to overcome both the overturning drag from the upward pitch as well as the gyroscopic stiffness of the bullet to keep it turning down into the tangent of the trajectory. So if the bullet were pitched up higher than that, it would precess inexorably toward a sideslip yaw of several times the pitch in response. Much more sideways than up. Since the drag trying to overturn the bullet increases as the square of the sine of the angle into the wind, this quickly results in tumbling at an angle slightly canted off horizontal before it ever actually got anywhere significantly off the trajectory tangent. Thus, except for a group opening up due to the bullet mass asymmetry causing wobble from spinning too fast, and absent keyholing, the bullet wasn't overstabilized. What was seen was some other effect, but it would be interesting to figure out what it was.

 

[JohnKSa]

I would be interested to hear what their experiences were.

Earlier in the thread I quoted one fellow who claimed that he was seeing consistent and aligned keyholes on a target when shot at distance.

... as I got more immersed in Robert L. McCoys seminal tome, Modern Exterior Ballistics, I realized the only possible outcome of being "overstabilized" to the point of being unable to remain point into the wind, is tumbling.

Nennstiel also quotes McCoy as a source and appears to recommend his book, but comes to a different conclusion than you do with regard to overstabilization.

 

[Llama Bob]

Earlier in the thread I quoted one fellow who claimed that he was seeing consistent and aligned keyholes on a target when shot at distance.

But we know that's not true or at least has a different cause, because we can calculate the angle of impact and it's too low (only a couple degrees for fast bottleneck cartridges) to show on paper.

I think what we have here is a good old fashioned myth. Shooters got the idea in their head that minimum sufficient twist was a good thing. And it was, in the bad old days of non-uniform jackets. But now we have much better bullets, and that reason is gone except for perhaps certain benchrest applications. But rather than getting rid of the obsolete min-twist idea, people go looking for a new reason why it must be right. There isn't one, but that doesn't stop people from finding one

 

[Unclenick]

If they saw keyholes, that would suggest to me that an instability occurred. The consistency makes it hard to swallow, but if they were hitting the target, the tumble would have to have just begun and not had time to move the bullet significantly off course, so the timing might be possible if they had low velocity SD's and consistent bullet BC's.

I realize my description of the bullet tumbling orientation to the side is misleading because it requires a pretty extreme upward nose pitch, not allowing the bullet nose time to circle before starting to tumble. If the pitch up is just past the stability margin, then precession will cause the nose to loop, and it may loop several times with increasing radius before tumbling initiates. So the orientation of the bullet at the start of the tumble could be anywhere around the clock in that case. In fact, that's got to be the more normal case since the rate of drop off the bore line grows as gravity accelerates the bullet downward. My sideways tumble will require a rapid increase in pitch, so I'll cover my face now and retract that aspect of the description.

If you fire a 175 grain SMK at 2700 fps to hit a 1200 yard target, angle of departure is about 1° and angle of fall is about 2°. In a vacuum, where the angle of departure would not be disturbed, you would see a 3° keyhole in the paper. I think Bob is right that you would be hard put to distinguish that from a straight entry into paper, what with the ragged edges of the hole and all. So even in a vacuum, this would have to be really, really long range or very low initial velocity or, in air, a high BC bullet that had to be fired with a high trajectory to hit the target.

Incidentally, in my example, the bullet turns through 3° in the 2.2 seconds it takes for the bullet to go 1200 yards to the target. That's a rate of 4 minutes, 24 seconds per complete rotation. Pretty slow.