If every gun tells a story, the FBI’s reference firearms collection could fill a very, very large book. The inventory of more than 7,000 firearms—curated over 80 years—contains just about every make and model, from John Dillinger’s Prohibition-era revolver to the modern battlefield’s M16 and almost everything in between.

Housed at the FBI Laboratory in Quantico, Virginia, the racks of weapons are not a musty exhibit of museum pieces, though some rare items would certainly qualify. Rather, the ever-expanding collection is a hands-on reference catalog for the Lab’s firearms examiners to study, take apart, reassemble, and test fire to support investigations. By maintaining a working library of virtually every handgun and rifle—along with a database of their unique toolmarks—examiners are able to identify and substantiate for investigators what weapons may have been used in criminal acts.

“Oftentimes, this collection is used in active cases in comparing known samples from our collection with question samples from the field,” said John Webb, a firearms examiner in the Lab’s Firearms/Toolmarks Unit. “Often, an investigator will receive a part of a firearm or a firearm that isn’t functional. We can take that and compare it with our reference collection, determine what isn’t functioning, and repair it so we can obtain the test fires we need to conduct examinations with bullets and cartridge cases.”

Most of the firearms come from closed investigations, though some are purchased and still others arrive as donations. In most closed cases, guns that were held as evidence in court are sent back to the Lab, where examiners can add them to the reference collection (whole or in parts) or have them destroyed. By continuously adding new pieces to the collection, the FBI aims to have a duplicate of every firearm. Sometimes a case could hinge on linking a firearm component to a similar part on a reference gun.

“The collection has been extremely useful in criminal cases, not only for an examiner’s experience and education in handling nearly every firearm case that comes into the Laboratory,” said Webb, “but it has been directly responsible for assisting to solve crimes.”