On a military base in Quantico, Virginia, the FBI keeps a collection of over 7,000 guns, including weapons used by John Dillinger and other famous American gangsters.

But these weapons aren't used in the field to prevent crime. Instead, they're part of a massive library used to help solve crimes.

With the weapons on hand, analysts can rebuild destroyed weapons needed for evidence and match ballistics from thousands of samples.

The BBC's Thomas Sparrow reports from the vaults.

Filmed and edited by Peter Murtaugh