DALLAS (AP) – A subtle design feature of the AR-15 rifle has raised a technical legal question that is derailing criminal cases tied to one of America’s most popular weapons. At issue is whether a key piece of the AR-15 satisfies the definition of a gun that prosecutors have long relied on. That definition is critical in pursuit of people who are charged with illegally buying and sell parts for the rifle or building them. Federal law enforcement officials have long been concerned about the issue. Now they are increasingly worried that it could derail some criminal charges and undermine firearms regulations nationwide.