Bullet microstamping, more correctly called cartridge case microstamping, is a technology which embeds information about a gun on each bullet it fires. The idea is that investigators at crime scenes could use the stamped bullet casings to track the owner of the gun, potentially reducing the amount of time needed to track down a suspect. Microstamping laws have been proposed in several states, usually specifically for hand guns, and they have met with considerable opposition from the gun rights community, for a variety of reasons.
In order for bullet microstamping to work, a weapon must be fitted with a special firing pin which has been etched with a unique serial number. When the gun is fired, the firing pin strikes the casing of the bullet, marking it with the serial number. While the microstamp is generally too small to read with the naked eye, it can be identified on a microscope, allowing investigators to cross-reference the serial number with a database of registered weapons.
There are several problems with bullet microstamping. In the first place, if a criminal picks up his or her shell casings, the microstamp will not be recoverable. For this reason, some people argue that microstamping would be more effective if it marked the actual bullet, not the casing. Microstamping also does not address the issue of stolen and unregistered weapons, and a large number of gun crimes are committed with such weapons. Gun owners could also potentially replace the firing pin or file the microstamp off to avoid microstamping when the gun is fired.
Firing pins wear over time, so microstamps will also change as wear occurs. Additionally, the wide range of hardness between primers, and between cartridge cases means microstamps will differ based on the brand of ammunition used, whether the cases are made of brass, nickel-plated brass or lacquered steel. And, the pressure of a given load will result in variances in microstamps. A .38 Special cartridge with a max pressure of 17,000 PSI will produce a different stamp than a .357 magnum cartridge at 35,000 PSI when fired from the same .357 magnum.
Advocates of bullet microstamping believe that even with all the arguments against it, it could still be of great help to forensic investigators. Every little bit helps when looking into a serious crime, so microstamping could generate a breakthrough in some cases. Opponents suggest that it could add substantially to the cost of new weapons, while also creating a tracking system for law-abiding gun owners who register their weapons, potentially abridging their right to privacy.
Even in regions where bullet microstamping laws have passed, no attempt has been made to make the laws retroactive, so numerous weapons would lack the microstamping technology. Making microstamping laws retroactive would be extremely expensive, time-consuming, and potentially impossible, so it is not surprising that no attempt has been made to expand the scope of these laws, but some people question the value of bullet microstamping legislation when used weapons which do not microstamp will be so readily available to those who want them.