The Defense Department's inspector general has criticized the Navy for a "completely inadequate" investigation of the Grenada gun-smuggling charges involving Vice Adm. Joseph Metcalf III and has accused the Atlantic Fleet commander of "poor judgment" in allowing Metcalf to stop the inquiry, a report released yesterday said.

The report by Inspector General Joseph H. Sherick said that Adm. Wesley L. McDonald, the Atlantic Fleet commander, was "remiss" in indicating to naval investigators that Metcalf was responsible for determining if there was need for an inquiry.

Navy investigators also were blamed in the 37-page report for failing to be "adequately aggressive in dealing with" Metcalf.

The report, culminating a five-month review by Sherick's office, was cited Friday by Navy Secretary John F. Lehman Jr. as evidence for exonerating Metcalf and concluding that McDonald had "acted properly" in handling the incident. Lehman was unavailable yesterday to discuss the apparent inconsistency between his statement about McDonald and the report's findings, but a Navy spokesman said Lehman stood by his comment.

Lehman had declined to release the report, a copy of which was obtained by The Washington Post yesterday after a formal request to the Pentagon.

Metcalf, who commanded the U.S. invasion of Grenada in October 1983, returned to Norfolk in a plane carrying 24 captured Soviet-made automatic rifles, including some tagged with his name or those of members of his staff. Federal and military regulations prohibit importing automatic weapons.

Sherick's office began its review in February after reports that Metcalf had received a nonpunitive "caution" while lower-ranking Marines and soldiers under his command were court-martialed and ordered to jail for up to three years for similar offenses.

The inspector general's report found that Metcalf, who claims to have taken the rifles as souvenirs to give to dignitaries, was "generally responsible" for the incident, but it focused blame on a member of his staff who tagged the weapons with the names of other officers, apparently without their knowledge.

The report's strongest criticisms was aimed at McDonald and the Naval Investigative Service (NIS), which received the case from Customs Service officials who seized the weapons on Metcalf's plane Nov. 3, 1983.

The NIS, then required to obtain a commander's request before launching an investigation, presented its findings to McDonald's legal adviser, the report said.

McDonald summoned Metcalf, who took responsibility for the incident, the report said. McDonald, reportedly satisfied that he understood Metcalf's role, told him to discuss the actions of his staff with NIS investigators, indicating that Metcalf should determine whether his aides should be investigated.

Meeting with NIS officers, the report said, Metcalf again assumed responsibility and, when asked if he wanted further inquiry, replied, "What is there to investigate?"

When NIS officers informed McDonald's counsel of their findings, they were told that no investigation of Metcalf was necessary. NIS dropped the matter.

"After Adm. McDonald concluded that Vice Adm. Metcalf was responsible for the incident," the report said, "it was poor judgment to allow Vice Adm. Metcalf to decide if there would be further investigation of the incident."

McDonald could not be reached for comment.