Here is a particularly woeful crime statistic from the Federal Bureau of Investigation: nationwide, nearly 40 percent of all homicides go unsolved each year. In California, the percentage of unsolved homicides is even higher, but a groundbreaking measure signed into law earlier this month by the state’s Republican governor, Arnold Schwarzenegger, offers a realistic hope for bringing more murderers, gang members and other armed criminals to justice.

Under the new measure  the Crime Gun Identification Act  manufacturers of semiautomatic weapons will have to equip new guns sold in California starting in 2010 with inexpensive technology known as microstamping. This process involves using lasers to create microscopic markings that record the make, model and serial number of a semiautomatic handgun onto its firing pin and other internal surfaces. These markings automatically transfer onto the bullet shell casing when a gun is fired, providing a valuable lead for police investigators when, as commonly happens, casings from the shooter’s weapon are found at a crime scene.

The new law, like any other single attempt to get America’s handgun crisis under control, is not a panacea. It will not help solve violent crimes committed with old weapons or with revolvers of any vintage. Gun casings are not always found at a crime scene, of course.

But a huge number of gun crimes are committed with the sort of semiautomatic handguns the law covers. Each new tool like this that is handed to law enforcement increases the chances of solving crimes; each tool denied them, like access to gun-sale records in other states, reduces those chances. This law’s value to law enforcement should grow over time as older guns are replaced by new ones equipped with microstamping. Meanwhile, it represents no inconvenience at all to law-abiding gun owners, and only a minor increase in the price of a weapon.