So the police sent the shotgun to the federal Bureau of Alcohol, Tobacco, Firearms and Explosives’ National Laboratory in Beltsville, where Gregory Klees, the lab’s go-to guy when it comes to the forensic science of un-erasing serial numbers, knew he had a challenge on his hands. The gun’s serial number, a forensic science staple for tracing guns, had been ground away. Several months ago, undercover police in Western Maryland bought a Mossberg shotgun, Model 500A, from a local drug dealer.
