The Reagan administration is considering supplying El Salvador with several rapid-fire AC47 gunships, the planes widely used in Vietnam, U.S. officials said today.

Use of the propeller-driven gunships, which carry small machine guns capable of firing up to 18,000 rounds per minute, would mark a technological step-up in the four-year-old war against leftist guerrillas seeking to overthrow the government.

The planes, nicknamed "Puff the Magic Dragon" in Vietnam, can concentrate up to 600 rounds per second within 50 square yards, withering whatever is there, according to military authorities.

Supply of the AC47s is being considered as part of a $70 million supplementary military aid package approved by Congress this month, according to William Schneider Jr., undersecretary of state for security assistance, science and technology.

U.S. Embassy officials questioned here refused comment on whether the planes would be supplied to El Salvador, but Ambassador Thomas Pickering told Reuter news agency they were under consideration as part of the new aid package.

U.S. diplomats said the $70 million also is likely to finance purchase of a dozen more UH1H Huey helicopters and a number of trucks. This is in line with eagerness by Salvadoran officers and their U.S. advisers to improve the Army's mobility and reaction time. The Salvadoran Air Force currently has 22 helicopters, including four earmarked for medical evacuation.

In all, the diplomats said, the package is likely to include $2 million for training additional soldiers, $19.8 million for supplies such as bullets, bombs and personal gear, and $48.2 million in new equipment. They emphasized, however, that the package is still being discussed and the mix could change.

The Salvadoran Air Force has five DC3 aircraft, a civilian transport version of the C47 that has been a mainstay of air forces around the world since it was introduced more than 40 years ago. Schneider said in Washington that three or four of these planes could be transformed into AC47 gunships in the United States in about six months at a cost of about $2 million apiece.

Politically sensitive because it would introduce new technology into the Salvadoran civil war, the AC47 was said by military observers here to be highly accurate. Increased accuracy could be a step toward meeting complaints from Salvadoran human rights groups and the leftist guerrilla leadership that bombing by Salvadoran A37 Dragonfly jets frequently causes casualties among civilians associating with guerrillas or simply caught nearby during an air attack.

The Farabundo Marti National Liberation Front, the overall guerrilla command, charged in a broadcast today, for example, that A37 bombing killed an elderly civilian and wounded two others in the La Ceiba and Las Lajas communities.

"The Air Force of the dictatorship continued in recent hours carrying out indiscriminate bombing against zones densely populated by civilians," the command said over its Radio Venceremos.

Such charges have been frequent during the past year, particularly since bombing has increased in the past six months. Salpress, the rebel news agency, reported a total of 165 civilian bombing incidents so far this year. There were 113 such incidents in the same period last year, according to the news agency.

But Secretary of Defense Caspar W. Weinberger said July 26 that he had received no reports of civilian deaths caused by such bombing.

The Salvadoran Air Force drops 750- or 500-pound iron bombs from its six U.S.-supplied A37 jets. U.S. and guerrilla officials, in separate conversations in recent days, have agreed the bombing is an effective weapon against rebel concentrations.

At the same time, U.S. advisers here have repeatedly insisted that the best way to confront El Salvador's guerrilla attacks is to send out frequent small patrols and respond in force once leftist units are contacted.

"This is not a big firepower play at all," said a source who insisted on being identified only as a military observer.

"It's more geared toward the basic, small-patrol counterinsurgency operation."

Aside from accuracy, the two-propeller AC47 plane has the advantage of being able to circle targets at low speeds. Sophisticated aiming devices aboard the aircraft can keep the machine guns trained on targets as the plane describes circles above them.

The rapid-fire machine guns can deliver so many bullets in a tight area that almost nothing is left intact where they have directed their fire.

With Salvadoran pilots routinely flying traditional DC3 and C47 transports, military observers here said, there would be little problem in training them to operate the planes.