Police say a man shot a Walmart drone. Armed Americans could pose a headache for air deliveries.

• A Florida man admitted to shooting a Walmart drone last week, law enforcement officials said.
• For the past decade, gun owners have been shooting at UAVs — in violation of Federal regulations.
• Now, as more retailers use drones, armed Americans may add further complication to delivery by air.

Retailers have had to solve a long list of technological, regulatory, and commercial challenges in order to offer deliveries by drone.

But one complication remains especially difficult to predict: US gun owners.