The forecasters inside and outside the defense industry are predicting good, but not boom, times for the nation's military-industrial complex in the 1980s.

Topping the list of reasons for this is the "new" Jimmy Carter.

In the space of a few months, the candidate who called for cutting the defense budget is now the President urging the following:

Higher defense budgets, so they increase by 4 to 5 percent a year after allowing for inflation, starting with $157 billion in new money and $142 billion in actual spending for fiscal 1981. (The comparative figures for fiscal 1980 are $139 billion and $127.4 billion.)

A much bigger Navy than he had recommended originally -- construction of 95 rather than 67 ships over the next five years.

More arms sales to foreign nations, a clear reversal, including lifting the 1977 ban on designing warplanes specifically for export.

Construction of planes to deliver the tanks and other armor for a newly organized quick reaction force of 100,000 troops designed to fight any place on the globe. They would be sustained in battle by the food and bullets stored inside a new fleet of cargo ships on the drawing board.

Delay of the strategic arms limitation treaty, known as SALT II, which sets limits on ocean-spanning bombers and missiles.

A "get tough" policy all around the world rather than fervent pursuit of "detente."

In short, the transformation of Jimmy Carter on defense issues is like a man who sings the praises of a compact car, only to suddenly junk it and buy a giant guzzler.

Despite all that, and contrary to the frenzied buying of defense stocks on Wall Street, the forecasters still refuse to say "boom" when talking about the 1980s. Before hearing their arguments consider these unchanging realities of the defense business:

Today's super-weapons cost so much a copy -- $2 billion for an aircraft carrier and up to $25 million for one fighter plane -- that relatively few of them can be bought each year, 5 percent increase in defense spending or not. This means many contractors get passed over when the billions are allocated.

Plans to buy new weapons often remain just that, with civilian leaders often losing their nerve and cancelling programs as they reach that phase that requires spending a lot of money.

Carter and the Congress already have launched a number of projects that will soak up much of the procurement budget in the near future, leaving the have and have-not situation largely unchanged. The $2 billion Nimitz aircraft carrier, $1 billion-a-copy Trident missile submarine, $33 billion MX missile programs and f15, f16 and f18 fighter programs are among the examples.

Interviews with a cross-section of executives from the top 10 defense contractors (in terms of prime contracts awarded in fiscal 1978) bought little more than cautious optimism.

One might expect that Litton's Ingalls shipyard in Pascagoula, Miss., would be ecstatic about Carter's new shipbuilding plan and other hawkish moves. It was Ingalls which built the Navy's new fleet 35 of Spruance destroyers and is a good bet to get the order for another two of those ships loaded down with Aegis armament for shooting down airplanes.

"All that is every encouraging," said Leonard Erb, president of Ingalls. "But you're looking at four or five years away" before Carter's new shipbuilding budget could mean bucks to Ingalls or any other yard, he said.

Even with the prospect of more Navy business than originally envisioned, Erb said: "I can't reasonably expect to keep this shipyard in a viable situation building only Navy ships." Therefore, he said, Ingalls is hedging its bet by building railroad cars and oil-drilling platforms.

When Ingalls was at its peak in 1978, 25,000 people were building the Spruance destroyers and LHA assault ships. Today, the work force is down to 14,000. By next year, said Erb is documenting how little faith Ingalls is putting in prediction of boom, the yard's is scheduled to drop to 8,500.

James M. Beggs, executive vice president for aerospace at General Dynamics, the nation's largest defense contractor, sounded the same theme of cautious optimism. "The year 1980 looks as good as last year and maybe better," Beggs said. General Dynamics' big-ticket items in terms of military sales will be the F16 fighter, Standard ship-launched and Singer shoulder launched anti-aircraft missiles for sure -- and maybe the Army's DIVAD gun for protecting tanks and other armor. General Dynamics is in the finals of that competition.

The company's prospects for selling the Air Force and Tomahawk air launched cruise missile also look good, he said. General Dynamics and Boeing are vying for that contract for about 3,000 missiles costing $3 billion. The Pentagon is expected to make its decision next month.