Robert Jones

Between tides, the water covering a reef was relatively still as a small pistol shrimp crept toward an even smaller shrimp. The pistol shrimp’s two claws had no pincers, but the larger of the two had a “pistol-like” device comprising two portions. A movable part was cocked like a hammer and then released to snap against a stationary part. The fast moving hammer caused a “tear” within a small volume of the surrounding water, causing the water to vaporize into a gas and create a tiny bubble, or cavity, in the water. The pressure of the surrounding water, then being much higher than that of the gas within the cavity, caused an immediate implosion of the cavity. The gas dissipated violently into the surrounding water, creating an acoustic shock wave that stunned the smaller shrimp. This allowed the pistol shrimp to secure what was to become lunch and drag it into a burrow it had made in the sand beneath the reef.

An hour later, the pistol shrimp was resting just outside its burrow and beside its constant, symbiotic partner – often referred to as a “watchman goby.” The shrimp’s antennae were resting upon the goby’s tail and body so that it could detect every movement of the fish. Having far better eyesight than its partner, the goby was the first to notice a large predator heading in their direction. Having no interest in exploring the inside of the predator’s stomach, the goby darted into the burrow it shared with the shrimp. Feeling the goby’s sudden movements, the shrimp assumed it had detected danger and ducked into the burrow with it. Their symbiotic relationship provided the shrimp with a lookout and provided the goby with a burrow in which to hide and to protect its eggs.

Tiny cavities are created in other situations when there are rapid changes in the pressures of small volumes of liquids. This is known as cavitation inception. When cavities are repeatedly imploded, they create impact stresses in contacting surfaces that result in wear of the surfaces. Such wear is known as cavitation. Cavitation processes come in two flavors: inertial (also known as transient) and noninertial. The process involving the pistol shrimp is inertial. Other inertial processes that include rapidly imploding cavities that produce shock waves commonly damage valves and boat propellers. In noninertial cavitation processes, a cavity is oscillated in size and/or shape, typically by an ultrasonic sound, when used in a cleaning bath. Cavitation may thus be understood to produce both beneficial and detrimental results.

While the pistol shrimp was enjoying its lunch and resting outside its burrow, a thief named Bernie was busy stealing a legally registered, unfired pistol during a B&E and grinding off its serial number. Following a tip to the police, Bernie was arrested under suspicion of having committed the B&E. The gun was found in his possession, and it became an important piece of evidence that linked him to the B&E. Without the serial number, however, how could investigators be certain the gun was the one stolen during the B&E … and what did that have to do with the pistol shrimp?

A common denominator between the pistol shrimp and the pistol serial number is cavitation. A pistol shrimp can use cavitation to get a meal, and a forensic examiner can often use cavitation to get a serial number from a pistol from which the number has been removed. Serial numbers (and/or letters) are typically pressure stamped into the metal of firearms. This not only displaces metal to form numbers, it displaces metal located below and beside the impressed numbers, rearranging its structure. To reveal the numbers, the area of the firearm that contained them is immersed in a fluid that is ultrasonically vibrated. The vibration causes cavitations that wear away the displaced metal faster than the nondisplaced metal, leaving in its place a cavitation-etched likeness of the original serial number. This procedure is known as the ultrasonic cavitation method.

Another restoration procedure is known as the chemical and electrochemical etching method. A forensic examiner using the method coats an area that previously bore a serial number with an etching solution. The solution etches the displaced metal faster than the nondisplaced metal surrounding it, revealing the original serial number. As indicated by the name of the procedure, the process can be accelerated by directing an electric current through the metal as well as by applying a chemical etching solution.

Yet another restoration procedure is known as the magnaflux method. This involves magnetizing the metal of a firearm and spraying it with an oil having iron particles suspended within it. Magnetic lines of force associated with a magnetic field created when the firearm was magnetized course through the metal, following contours of the metal. The iron particles in the oil align with the lines of force. The shapes of lines of force passing through displaced metal will follow the contours of the displaced metal, and the iron particle disposition will provide a copy of the original serial number that is visible to an examiner. This method has an advantage of being nondestructive, that is, it finishes with all the evidence it began with.

The serial number restoration method applied depends on the type of material used to make a firearm in question, how a serial number was originally applied and what was done while attempting to obliterate the serial number. In addition to the three methods of serial number determination described, sometimes simpler methods are all that are required. These include optically scanning the area that previously bore a serial number, perhaps after applying chalk to accentuate any remaining portions, and observing the area using various light sources.

Oh yes, Bernie will not be grinding any serial numbers for some time.

ADDITIONAL FACTS:

The snapping sound created by a pistol shrimp has a duration of less than a millisecond; but it compares in loudness (reportedly 218 decibels, which is twice as loud as a chain saw positioned a yard away) to sounds made by sperm and beluga whales. This makes the shrimp one of the loudest animals in the sea. The pressure of a sound wave emitted by a pistol shrimp is reportedly powerful enough to kill small fish and break glass, and the noise made by groups of pistol shrimp is loud enough to interfere with sonar and undersea communications.

When a cavity implodes as described in the foregoing, the pressure within it can reach several hundred atmospheres; and the temperature within it can reach several thousand degrees Fahrenheit,

It is unlawful to remove or change the serial number of a firearm.

As indicated in the foregoing, firearm serial numbers are not all restricted to actual numbers. Some also include letters.

The serial numbers of firearms made by one manufacturer are all different, but the numbers used by one manufacturer are sometimes the same as those used by another.