The Department of Defense recently uploaded a short, oddly soundless video of the National Firearm Reference Vault at the ATF’s facility in West Virginia. The reference collection is mostly seizures from criminal cases, some firearms surrendered to the ATF, and a few items sent directly to the agency by gun makers and importers. Since the collection is not open to the public, this video is one of the few rare looks an outsider will get of one of the largest firearm collections owned by the government.
Keep in mind that it is a reference library, and not a collection on display in a museum, so the vault is plainly adorned and firearms listed alphabetically and by country of origin.
The FBI also has its own reference collection. You can learn learn more about that vault below: