Rep. Clarence D. Long (D-Md.), chairman of a key House subcommittee, said today he will introduce a motion next week to prevent $5 million in U.S. military aid from being used for U.S. advisers in El Salvador.

"I feel even more strongly than when I left Washington," said Long after about 30 hours here, "that we should not have U.S. military advisers" in this Central American trouble spot.

Minutes after 72-year-old legislator's press conference at the U.S. Embassy here, the building was strafed with bullets fired from a passing pickup truck.

Long had left the area by the time of the shooting at 1:05 p.m. local time. No one was injured by the 14 bullets fired from what embassy security officials believe was a 9 mm automatic weapon, possibly an Israeli-made Uzi submachine gun.

It was not clear whether the incident was related to Long's visit. It was the second such shooting in a month.

Long is the chairman of the House Appropriations subcommittee on foreign operations, which is being asked by the administration to give its blessing to the reallocation of $5 million of military sales credits that the administration wants to include in a $25 million package of military support for the Salvadoren government this year.

Long said that he will introduce a motion that "none of the $5 million be used for trainers and advisers in El Salvador" when he returns to Washington next week after meeting with leaders of the Salvadoran left in Costa Rica and Mexico.

[The House subcommittee vote is expected Tuesday, and sources in Washington indicated that there could also be a motion to defeat the administration's request outright. A close vote is expected in any event. The Senate appropriations subcommittee voted to support the reallocation Monday.]

Although several congressmen have talked about coming here since El Salvador became the subject of furious debate on Capitol Hill, Long is the only one who has.

His major concerns are the more than 50 U.S. advisers serving here and the future of the Salvadoran government's land reform program, which is designed to help win support away from the guerrillas seeking its overthrow.

The congressman suggested at his press conference that President Reagan would do well to read up on the past experience of the United States in Vietnam and Central America. "I wish to God a president of the United States would read a few books for a change," said Long.

It is Long's opinion that the administration may be looking to Congress for approval of the $5 million to get Capitol Hill's endorsement of its policy of increasing the U.S. military presence here.

"It may be that what they are really looking for is a kind of Gulf of Tonkin resolution," Long said, referring to the congressional vote that opened the door for massive U.S. involvement in Southeast Asia.

"You put yourself in a situation where events take charge," said Long. "The great disadvantage" of growing U.S. involvement here is that it would tend, as it did with the Vietnamese, to weaken the Salvadoran's capacity to take care of themselves rather than strengthen it."

Long said he is convinced that Salvadoran troops can be trained adequately outside of El Salvador and that such a program would eliminate the current risk of shedding American blood here.

Long, whose son was wounded in Vietnam, considers that risk substantial.

Long went ahead with a scheduled trip to some of the farms affected by agrarian reform this afternoon despite the firing on the embassy grounds.

At his press conference Long said he was enthusiastic about the agrarian reform, especially that phase that is supposed to give sharecroppers the land they worked. But he expressed doubts about the collectives that have been set up on more than 280 large farms seized by the government in the last year. "I think cooperatives are 2 nothing more than socialism . . . communism," he said.