Federal law generally prohibits firearm dealers from importing machine guns unless they are being used as demonstration models for law enforcement agencies, which can legally purchase such weapons. But in order for dealers to use that exemption, the interested law enforcement agency must submit a letter to ATF requesting a sample of the machine gun they are considering purchasing.

According to the indictment, Jenkins wrote such letters on behalf of his co-defendant, Robert Krop, whose Frederick County business The Machine Gun Nest offered machine guns for rent. The business made over $100,000 in profits from the rentals in 2018 and 2019 alone, the indictment says.

https://www.sandiegouniontribune.com/news/nation-world/story/2023-04-12/maryland-sheriff-charged-in-illegal-gun-rental-scheme