The Army's controversial Divad air defense gun failed to prove its worth in recent tests that Defense Secretary Caspar W. Weinberger ordered to determine the future of the $6 billion weapons program, according to a congressman who reviewed videotapes of the tests.

Rep. Denny Smith (R-Ore.) said in a letter to the Pentagon's testing director that no Divad "ever made a single direct hit on any of the targets at any point during the tests."

The Army responded yesterday that "in virtually every case" proximity rounds fired from the Divad exploded so close to the target drones that real helicopters or airplanes would have been destroyed in combat.

Weinberger is to be briefed on the tests Aug. 23 and has promised Congress a decision by Aug. 30. In a recent letter to Rep. Joseph P. Addabbo (D-N.Y), he called the tests "the most realistic operational testing that we have ever put a weapon system through."

But Smith noted that employes of Ford Aerospace & Communications Co., Divad's manufacturer, "were intimately involved" in the tests, casting doubt on their realism. He said that target drones did not maneuver evasively and were destroyed before damage was determined.