Interesting information. I didn't realize that plain water is a better tissue simulant than Clear Gel.
It is.
A strict technical approach to establishing the dynamic similitude of proposed tissue simulants to human soft tissues requires that their respective EOS (equations of state) match to correctly represent nonlinear impact responses to projectile strikes. For fluids and soft solids (that are composed primarily of water), EOS is expressed as the Hugoniot shock velocity (U). Hugoniot shock velocity is the velocity of the wave that is created by, and preceeds, a projectile as it passes through a substance. Hugoniot shock velocity (U) is a function of the material's mass density (ρ), a first-order derivative of the material's bulk modulus (K'), the material's internal sonic velocity (c) and the particle velocity (v) most usually expressed in the form of a linear function; U = c + K'v
Materials that have coinciding EOSs (equivalent or nearly equivalent intercepts and slopes) are taken to be dynamically equivalent to one another.
For the materials currently under discussion, the respective EOS are:
Human soft tissues (composite): U = 1.561 + 2.160v
10% Type 250-A ordnance gelatin: U = 1.513 + 2.024v
Water: U = 1.497 + 1.901v
Clear Ballistics Gel: U = 1.434 + 1.444v
Since the slope of the Clear Ballistics Gel EOS differs greatly from the EOS of the two proven soft tissue simulants and the human soft tissue composite profile, data obtained in the Clear Ballistics Gel product will deviate significantly from them at both extrema along any imaginable terminal penetration curve. The CBG EOS indicates that it is significantly different from the other three substances by a wide margin.
Furthermore, the Clear Ballistics Gel product introduces two other confounding factors that detract from its ability to correctly simulate human soft tissues.
First, projectiles tested in the Clear Ballistics Gel product tend to exhibit disproportionate amounts of terminal rebound. All projectiles rebound in both human soft tissue and 10% ordnance gelatin, but only at very small scales on the order of larger fractions of an inch. In water, because it does not support shear, no rebound occurs at all. However, in Clear Ballistics Gel, projectiles can, and often do, exhibit tremendous terminal rebound sometimes as much as 25% - 30% of the entire permanent channel length confounding test data in the process.
The 'rebound' phenomenon is demonstrated in this video by
ShootingTheBull410:
45ACP vs 454 Casull - Raging Judge Magnum in ClearBallistics ballistic gelatin - YouTube
At 8 seconds in the slow-motion video, the projectile tested in the Clear Ballistics Gel product can be observed at its maximum terminal penetration depth (≈18 inches)
Shortly thereafter in the slow-motion video (about 9 seconds), the projectile can be observed having rebounded approximately 4½ inches rearward from its maximum terminal penetration depth of 18 inches, or about 25% of its maximum depth. The rheological properties of the Clear Ballistics Gel differ greatly from soft tissue and the proven soft tissue simulants causing the temporary cavity to remain near its maximum for an extended period of time allowing the projectile to oscillate within the cavity coming to rest only when the temporary cavity finally collapses. This unusual behavior obviously introduces additional significant error into the test result.
Finally, the Clear Ballistcs Gel product commonly exhibits behavior during penetration events that is never seen in either of the two proven soft tissue simulants or in human soft tissues. In this slow-motion video by
The Wound Channel—
Incredible Super Slow Motion Bullet Impact! - M855A1 - YouTube
—the collapsing temporary cavity, seen at 36 seconds, causes the volatiles from the Paralux-701 paraffinic processing oil used in the manufacture of the Clear Ballistics Gel product to auto-ignite under adiabatic compression (also known as ''dieseling'').
This effect is often attributed to sonoluminescence, but sonoluminescence is not a process that involves combustion. The sooty, black smoky residue being ejected from the projectile's point of entry in the Clear Ballistics Gel block is indicative of an exothermic process—that is, combustion—involving the volatiles liberated from the Paralux-701 in the Clear Ballistics Gel during the low-pressure phase of cavity evolution and is the result of the incomplete combustion of those volatiles during the ejection of the heated combustion products (as unburnt carbon in the smoke) from the block (as seen below).
The auto-ignition temperature of Paralux-701 is 358°F. This means that the temperature inside the collapsing Clear Ballistics Gel's temporary cavity is at least 358°F and is likely somewhat higher than that. This effect does not occur in human bodies that are struck and penetrated by munitions and serves as further evidence that the Clear Ballistics Gel product is not a correct/sufficient representative analog for human soft tissues of any sort.
I guess the main problem with water is trying to get consistent measurements of penetration without making a huge mess.
Since water doesn't support shear, maximum terminal penetration depths are easily computed using any of the three bullet penetration equations in existence. The WTI formula (1994) and the Q-Model (2012) are Poncelet forms both of which are strongly correlated (respectively) against 400+ and 900+ gelatin data across the spectrum of calibers, velocities, and sectional density. The
m-THOR algorithm (2014), also highly correlated against 900+ gelatin data, is a heavily modified 1950s-era armor SLV (Survivability, Lethality, Vulnerability) equation that can also be used to predict maximum terminal penetration depth with a high degree of accuracy and confidence.
Fackler's conversion factor for the conversion of penetration depth in water to equivalent maximum terminal penetration depth in 10% ordnance gelatin, is a simple linear conversion value of 1.5x (IWBA Fall 2001 5;2 page 21) that, while well intentioned, introduces significant error into estimates of penetration depth since it treats the process as a simple linear function. It is not.
Best practice at this time is to use four to six ½-gallon water-filled paperboard cartons lined up and backed by old towels to capture the expanded projectile should it escape the test arrangement.
A mess? Kind of...but, in the summer, who doesn't mind a refreshing splash of water?!
Keep those chronographs covered.
