Norfolk Police has launched an internal investigation after it lost two magazines of ammunition which were accidentally left on the roof of a police car.
The force said the Postwick Interchange on the A47 near Norwich was closed on Friday at about 15:30 BST for more than two hours after the ammunition fell on to the slip road.
"Police equipment was accidentally left on the roof of a police car while officers were responding to back up colleagues attending a firearms incident," it said.
The road was reopened at about 18:00 BST.
The force said a review of incident was being dealt with "promptly" by senior managers.
"Two magazines containing ammunition fell into the area of the slip road so officers contained the area to ensure the safety of the public," it said.
"The equipment was subsequently safely recovered and the road reopened."
