A group of congressional liberals is preparing a letter urging Secretary of State George P. Shultz not to supply El Salvador's Air Force with AC47 gunships capable of firing 18,000 rounds a minute.

The Reagan administration has said it is considering sending three of the planes, nicknamed "Puff the Magic Dragon" in the Vietnam war, to help the Salvadoran military in its campaign against leftist guerrillas. State Department officials said yesterday that a decision has not yet been made.

Rep. Ted Weiss (D-N.Y.) released a draft letter yesterday expressing concern that Salvadoran pilots would be unable to distinguish between guerrillas and civilians and arguing that the use of AC47s thus could "increase civilian casualties."

A spokesman for Weiss said eight House members had signed the letter as of yesterday.

He added that the draft is being circulated in hopes of obtaining more signatures before it is sent to Shultz next week.

U.S. officials said there are differences of opinion within the administration about whether to give El Salvador the low-flying, twin-prop planes equipped with three rapid-fire guns that have been described as capable of putting a bullet in every square foot of an area the size of a football field.

Early this year, Nestor D. Sanchez, a deputy assistant secretary of defense, expressed reluctance about sending heavy-firepower gunships to El Salvador "because of our concern over atrocities."

But the use of U.S.-supplied jet bombers against the guerrillas has been criticized for causing heavy casualties among civilians near combat zones, and some U.S. officials argue that using gunships such as the AC47 would lessen the risk to civilians because their firepower can be controlled more precisely.