Identifying the firearm used in a crime is one of the biggest challenges for criminal investigators. But what if a shell casing picked up at a murder scene could immediately be tracked to the gun that fired it?

A technique that uses laser technology and stamps a numeric code on shell casings can do just that. But the technology, called microstamping, has been swept up in the larger national debate over gun laws and Second Amendment rights, and efforts to require gun makers to use it have stalled across the nation.

“I think it is one of these things in law enforcement that would just take us from the Stone Age to the jet age in an instant,” said Commissioner Frederick Bealefeld III of the Baltimore Police Department. “I just can’t comprehend the opposition to it.”

But legislation proposed in several states to require manufacturers of semiautomatic weapons to use the technology has met with fierce opposition. Opponents, including the gun industry and the National Rifle Association, argue that microstamping is ineffective and its cost prohibitive. They say the proposed system would unfairly focus on legal gun owners when most crimes are committed with illegally obtained guns.