More scaremongering about centerfire .22's...

[benEzra]

21. Well, to get pedantic...

You have strayed a bit from explaining why the least powerful and rarely misused civilian rifles are ZOMG THE MOST DEADLIEST EVAR, but if you want to get pedantic....

You said:

"For something to cavitate is to cause cavitation - to itself. Cavitate is an *intransitive*verb as you are using it, where the verb does not apply to a direct object."

Either usage is correct, and both are common in the peer reviewed fluid dynamics literature. A cavitating/supercavitating projectile creates a creates a temporary cavity in the fluid medium, not in itself. Yes, cavitating/supercavitating can refer to the fluid flow (especially when considering a system in the rest frame of the solid object, with the surrounding fluid portrayed as moving), but it is also correct to refer to the projectile as cavitating/supercavitating. In both cases, the temporary cavity is created in the fluid medium. For example:

Owis FM, Nayfeh AH, "Numerical simulation of 3-D incompressible, multi-phase flows over cavitating projectiles", European Journal of Mechanics - B/Fluids 23:2, Mar-Apr 2004 pp. 339-351 (http://www.sciencedirect.com/science/article/pii/S0997754603001171).

Kulkarni SS, Pratap R, "Studies on the dynamics of a supercavitating projectile", Applied Mathematical Modeling 24:2 (1 Feb 2000), pp. 113-129. (http://www.sciencedirect.com/science/article/pii/S0307904X99000281)

Rand R, Pratap R, Ramani D, Cipolla J, Kirschner I, "Impact Dynamics of a Supercavitating Underwater Projectile". Sacramento, CA: Proceedings of the 1997 ASME Design Engineering Technical Conferences, 1997. (http://www.math.cornell.edu/~rand/randpdf/ahsum.pdf)

Ditto cavitating/supercavitating hydrofoils:

http://www.globalsecurity.org/military/systems/ship/hydrofoil-limits.htm
"In order to prevent damage by cavitation, foils referred to as supercavitating foils have been developed. With a supercavitating foil, a large vapor-filled cavity, referred to as a separation bubble, is formed over substantially the entire upper surface of the foil. Vapor bubbles in the cavity are carried beyond the trailing edge of the foil and collapse in the water aft of the foil, so that shock waves produced by the collapse of the bubbles have much less effect on the foil than in a normal cavitating foil."

Baker ES, Notes On the Design of Two Supercavitating Hydrofoils. Bethesda, MD: Naval Ship Development Center, 1975 (http://www.dtic.mil/dtic/tr/fulltext/u2/a030749.pdf).

The usage isn't new, and goes back to at least the 1960s, to the very beginning of computational fluid dynamics. That doesn't mean your use of the term is wrong, either, it's just a different usage, and you see both ways in the fluid dynamics literature.

More broadly, I think your resistance to seeing the temporary cavity as a physics/fluid dynamics phenomenon (and therefore understandable and predictable) stems from the misconception of reading articles and definitions about low-order cavitation (e.g. microscopic cavitation bubbles, small-area sheet cavitation, prop-tip vortex cavitation, pump impeller cavitation, etc. that occur at relatively low Reynolds numbers/high cavitation numbers and are a major engineering headache), and thinking that's all there is to the study of cavitation. Fully developed large-scale cavitation structures, like the attached cavity formed by a bullet in a fluid medium, are described by the same equations and principles; the difference is simply the velocity of the relative flow between the projectile/object and the fluid medium.

The thing is, transition from slow-speed bubble cavitation to fully developed attached-cavity cavitation (where the temporary cavity is much larger than the projectile creating it, as with firearm projectiles) occurs at no more than a few dozen meters per second for a bullet-sized object; any velocity greater than that will produce a fully developed, vapor filled temporary cavity. Since all bullets are traveling between 250 and 1500 meters/sec, they are all waaaaay above the velocity threshold to undergo boundary layer separation and fully developed cavitating flow (e.g. supercavitation) and produce an attached temporary cavity far larger than the projectile itself. In mathematical terms, the cavitation number << the critical value (calculated thusly).

"Bullets do not cavitate, (unless they are mechanically affected which they cannot in less than a second), they cause cavitation due their mass, velocity, rotation or perhaps tumble."

They *do* cavitate---or if you prefer, cause cavitation; either use is acceptable---and it is because of their velocity and the energy they impart to the fluid medium; in a nutshell, they push the medium out of the way so fast that the medium's momentum moves it further than the diameter of the bullet, and it takes a few milliseconds for the displaced fluid to reverse direction and collapse the cavity. The shape and toughness of the bullet determine the shape of the temporary cavity, but the energy of the bullet determines the size.

But here are the more important points that you have gotten away from, with regard to civilian ownership of centerfire .22's:

(1) All bullets produce a temporary cavity, regardless of what you call it.

(2) .223 Remington produces a larger temporary cavity than most handgun bullets.

(3) .223 Remington produces a smaller temporary cavity than most rifle bullets.

(4) Temporary cavity size is not particularly associated with lethality, but at rifle velocities seems to be associated with temporary incapacitation due to transient vascular pressure effects and the body's (presumed) vasodepressor/vasovagal response. Some researchers have argued for temporary incapacitation effects with some handgun rounds also, but the evidence is inconclusive.

(5) Permanent cavity size is highly associated with lethality.

"There is a diff between cavitation of metallic parts & cavitation used in wound ballistics. The cavitation noted below is NOT what we are concerned with. We are not in an engineering context discussing temporary wound cavities, we are in a medical context."

If you fall off a ladder and break a bone, the consequences of the fracture are medical, but Newtonian physics and materials science/engineering tell you how and why the bone broke, can predict the compression/torsion/shear that a particular bone can withstand before breaking, and can tell you what kinds of accidents can generate those forces.

Likewise, if a high speed projectile enters a fluid medium---whether water, ballistic gelatin, or flesh---the consequences of a wound can be medical, and are mostly associated with the permanent cavity. But the impact itself, and the formation of the temporary cavity, are described by high Reynolds number fluid dynamics, not medicine. Temporary cavity is a mathematically describable physical phenomenon, and is determined primarily by the kinetic energy, shape, and deformation of the upset projectile.

"Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. This results in surface fatigue of the metal causing a type of wear also called "cavitation". The most common examples of this kind of wear are to pump impellers, and bends where a sudden change in the direction of liquid occurs. https://en.wikipedia.org/wiki/Cavitation"

Again, you are confusing low-order bubble cavitation (lower Reynolds number, higher cavitation number) with high-order, fully developed attached-cavity boundary layer separation (very high Reynolds number, very low cavitation number). It's still cavitation, but at the high-energy end of the spectrum (aka supercavitation), and you don't seem to be able to grok that they are two ends of the same continuum.

"ezra: due to low-order cavitation and subsequent bubble collapse on the surface of the cavitating object, causing metal fatigue.
jimmy: That is irrelevant to very-high-speed objects like bullets."

Yes, that's what I'm saying. High speed cavitation involves the formation of attached vapor-filled cavities, e.g. the temporary cavity, not tiny bubbles.

"No, the bullet itself does not form any cavity. The cavity formed is the tissue cavitating, not the bullet. As I've said previous, 'cavitate' has morphed into being accepted as you are using it, bullet wise, tho I believe it is invalid usage."

The bullet does indeed create the temporary cavity. Here's a larger-caliber Hornady deer hunting bullet doing just that, in ballistic gelatin:

.223 bullets just happen to create a smaller temporary cavity (and more importantly for hunting, a smaller permanent cavity) than more powerful rifle rounds do, all else being equal.