The House and Senate Armed Services Committees began inquiries yesterday about why a Navy vice admiral was not punished as harshly as lower-ranking soldiers and Marines for bringing Soviet-made rifles home after the invasion of Grenada.

Meanwhile, Navy Secretary John F. Lehman Jr. defended the simple, nonpunitive "caution" given to Vice Adm. Joseph Metcalf III as "punishment that fits the crime" and said he was responsible for halting investigation of Metcalf.

The committees were responding to reports that five soldiers and two Marines were court-martialed and ordered to serve jail terms as long as three years for bringing back Soviet-made AK47 automatic assault rifles. The Navy and Army were asked to provide case records for the congressional attempt to determine if Metcalf, who commanded U.S. troops in Grenada, received preferential treatment.

"If they're going to give that admiral a tip of the hand, that's okay," said Rep. William L. Dickinson (R-Ala.), ranking minority member of the House committee. "But they're sure not going to railroad those soldiers."

Congressional investigators said one area of interest is the disclosure that Metcalf played a role in quashing the Navy's investigation of his case.

According to a Navy document, naval investigators dropped their inquiries after Metcalf admitted responsibility for bringing back more than a dozen of the rifles for himself and his staff despite military and federal prohibitions, then asked, "What is there to investigate?"

Metcalf was confronted by naval investigators after they asked his commanding officer, Atlantic Fleet Commander Adm. Wesley McDonald, how to proceed and were told that "Metcalf requested that he be allowed to handle the problem himself."

Metcalf's involvement in his own investigation was questioned at the time by Defense Department Inspector General Joseph Sherick, who wrote a memo to Lehman. Dated Jan. 15, 1984, it asserted that "the authority to request investigation of the matter by the Naval Investigative Service was delegated to the subject of the investigation.

"I find this to be particularly unacceptable," wrote Sherick, whose congressional mandate is independent investigation of alleged wrongdoing in the Pentagon. "I would like to have the Navy's assessment of the propriety of such a procedure."

In an interview, Lehman confirmed the memo's contents and said that, while Metcalf technically could tell investigators they had nothing to probe, the final decision was up to Lehman.

Lehman said that investigators reported their findings to him at the time and that, after his review, he concluded that "there was nothing more to investigate."

He said Metcalf admitted wrongdoing and had received a letter of "caution" from his commanding officer at Atlantic Fleet headquarters. "I never thought there was anything left to be done," Lehman said.

"Metcalf couldn't halt the investigation," Lehman said. "No commander can stop an investigation by the Naval Investigative Services. It works directly for the secretary and chief of naval operations."

Lehman said Metcalf's case should not be confused with that of some of the lower-ranking troops who not only brought back similar weapons but sold them on the black market.

He said Metcalf was "hit harder" than other servicemen who voluntarily gave up contraband during an amnesty program and were not punished.

The committee inquiries coincide with renewed Pentagon investigation of the cases.

The committees' staffs were instructed to have the Army and Navy detail all cases involving weapons taken from Grenada. Their aim, according to committee sources, is to determine whether the lower-ranking troops, one of whom was an officer, were penalized too harshly and Metcalf too lightly.

"Rank should not determine justice," House committee member Rep. Patricia Schroeder (D-Colo.) said. "The Metcalf case is not rare. It is just one that illustrates the disparity of justice within the ranks and within the services."

Senate committee member J. James Exon (D-Neb.) said the inquiry should determine "whether the officers are being as severely dealt with as the enlisted men.

"Certainly, the Uniform Code of Military Justice should be applied in a uniform manner to every man and woman in the military."