One year ago, Lawrence J. Korb was one of the Reagan administration's most visible point men in the debate over military spending, an assistant secretary of defense who appeared before dozens of congressional committees to urge support for Defense Secretary Caspar W. Weinberger's arms buildup.

Yesterday, Korb was back on Capitol Hill -- this time to oppose the administration's military budget request and appeal for a reordering of priorities.

Now a private citizen working for arms maker Raytheon Co., Korb spoke as a member of the Committee for National Security, a group of defense experts that yesterday released a report titled "Defense Choices: Greater Security With Fewer Dollars." The group, which includes former arms control negotiator Paul Warnke and William W. Kaufmann, an adviser to six past secretaries of defense, said that the Pentagon can pare $200 billion from its five-year spending plan and emerge nonetheless with a stronger force.

"The threats cited most consistently by the Reagan administration to justify its buildup have either not materialized, or proved far less menacing than advertised," the report said.

Korb explained at a news conference that his debut as administration critic did not suggest a repudiation of past administration efforts, but a determination to protect the "excellent gains we've made in the past five years."

He said that, in a time of rising budgets, the Pentagon's long hardware shopping list had made sense. Now, he said, with budgets unlikely to rise, the military should consolidate its gains to ensure that it does not end up with a "hollow" force of tanks, ships and planes without the personnel or spare parts to keep them running.

Korb's arguments made public a debate that he reportedly waged in the Pentagon during his tenure as assistant secretary for manpower, logistics and reserve affairs. As the "readiness" czar of the Pentagon, Korb frequently tangled with Navy Secretary John F. Lehman Jr. and others who wanted more ships and weapons at the expense of manpower and spare parts. Yesterday, Korb was urging a scaling back of Lehman's 600-ship, 15-carrier-group Navy.

President Reagan is seeking an 8 percent increase, plus an allowance for inflation, for the Pentagon's fiscal 1987 budget. Over the next five years, he wants $1.77 trillion for the military, which equals $1.501 trillion in 1986 adjusted-for-inflation dollars.

The Committee for National Security crafted three alternative budgets: a recommended plan that would allow growth only for inflation and would cost $1.305 trillion over five years in 1986 dollars; a plan that would not allow for inflation and would cost $1.157 trillion, and a plan that would meet the Gramm-Rudman-Hollings spending targets and would cost just under $1 trillion.

Even the third option, which would reduce military spending to levels lower than those inherited by Reagan, would allow the United States to "maintain a strong and survivable deterrent," the report concluded.

But the committee recommended the $1.305 trillion budget, which -- with judicious trims -- "would actually perform better than the force the administration proposes to buy," the report said.

The recommended savings would slow "the excessive pace of procurement that has characterized" the buildup, the committee said. Korb said pruning unneeded weapons would save far more than the currently popular procurement reforms.

"Buying a wrong weapon more cheaply is not much of a bargain either," he said.

Among Kaufmann's candidates for the ax are the proposed Stealth bomber, which would be replaced by more B1s; the proposed C17 cargo plane, which would be replaced by more ships and cheaper C130s; the proposed Midgetman nuclear missile; the Navy's next-generation Seawolf attack submarine, and other weapons that are just about to leave the drawing board to go into production.