A Senate-passed bill to tighten Defense Department accountability for large cost overruns on major weapons systems has stirred behind-the-scenes opposition in the Pentagon and may be weakened in a House-Senate conference.

The bill says that if the cost of a weapons system, including inflation, exceeds a baseline by more than 15 percent during development or 10 percent during procurement, the Pentagon must explain the reasons to Congress within 30 days, in a report that is to include the names of the responsible officials and the plans to curb further overruns. The baseline would be the cost estimated by the Pentagon in its March 31, 1981, SAR (selected acquisition report). The SARs, which are filed quarterly, describe the current status and cost of nearly 50 major weapons systems.

The bill would bar further outlays for a system if the Pentagon's report is filed more than 30 days after its due date.

The Senate passed the bill on a lopsided 96-to-0 vote last May, as an amendment to the 1982 Defense Authorization Act, which is now in House-Senate conference. The House has adopted no similar proposal.

Officially, the Pentagon takes a noncommittal approach to the amendment, saying it "has reviewed and is in the process of working with congressional staff to make the cost reporting system more effective."

But Sen. Sam Nunn (D-Ga.), the amendment's sponsor, said in an interview yesterday that although some Pentagon representatives were negotiating changes in the amendment's terms in apparent good faith, others "were working with House staff to oppose the amendment completely."

Rep. William L. Dickinson (Ala.), a leading foe of the amendment in the House-Senate conference and the ranking Republican on the House Armed Services Committee, also acknowledged flat-out Pentagon opposition in an interview.

In addition, strong objections to the amendment were made by Defense Secretary Caspar W. Weinberger in a July 21 letter to Sen. John G. Tower (R-Tex.), chairman of the Senate Armed Services Committee. Tower, after criticizing the amendment on the Senate floor, voted for it anyway, saying, "You cannot vote against motherhood . . . ."

The Project on Military Procurement, a nonprofit watchdog group, has obtained unsigned memos, apparently written in the Pentagon in June, suggesting key changes in the amendment's wording, such as excluding inflation in assessing overruns and eliminating the requirement for written explanations. A former Defense official consulted by the project said that the changes would devastate "the only recent action that actually discourages overruns."

It was only six years ago that Congress began to require the Pentagon to submit the quarterly SARs. In a 1980 report, however, the General Accounting Office said that the report omitted "important information," almost always including the original cost estimates that the Pentagon uses to sell Congress on weapons systems. Displaying an SAR on the Senate floor, Nunn said that much of it was classified, and "almost all of it is unintelligible."

He said the amendment is "strong action to control . . . dramatic cost growths" and would "begin to introduce Congress to the real world of defense procurement."

Amendment foe Dickinson asserted, however, that it "creates a lot more mischief than it would help." Singling out a requirement that the secretary of defense certify any increase of 25 percent or more over baseline, he asserted--but amendment supporters denied--that inflation for which the secretary bears no responsibility could cause a system to be terminated.

Both Nunn and Rep. Newt Gingrich (R-Ga.), a member of the House-Senate bipartisan Military Reform Caucus, warned that defeat of the amendment would increase congressional and public disillusionment with the Pentagon's ability to get a handle on overruns and, in the long run, would have detrimental effects on national security.

Gingrich said that if the conferees reject the amendment, he will favor a slash of several billion dollars in the 1982 defense budget. "If the Pentagon is in effect lobbying against the amendment, they are avoiding short-term problems while building long-term crises for themselves," he said.