A subtle design feature of the AR-15 rifle has raised a technical legal question that is derailing cases against people who are charged with illegally buying and selling the gun's parts or building the weapon.
For decades, the federal government has treated a mechanism called the lower receiver as the essential piece of the semiautomatic rifle, which has been used in some of the nation's deadliest mass shootings.
Federal law enforcement officials, who have long been concerned about the discrepancy, are increasingly worried that it could hinder some criminal prosecutions and undermine firearms regulations nationwide.