Authorities said the men drove a U-Haul van to the UPS warehouse in Memphis Sunday, where they stole about 400 guns.
The Bureau of Alcohol, Tobacco, Firearms and Explosives is investigating and offering a $5,000 reward for information leading to arrests and convictions.
Authorities are also investigating whether the theft was an inside job.
The exact number and type of guns stolen is still being determined and investigators are still working on a timeline for the crime.
The guns were being shipped to various cities and companies.