Despite a projected shortage of more than 50,000 GI helmets, the military last summer scrapped -- and in some cases "demilitarized" with an acetylene torch -- usable Army helmets in good condition, according to Pentagon investigators.

An unknown number of helmets were scrapped in West Germany in violation of an order from Washington banning the disposal of any surplus military property. That supposedly comprehensive freeze was imposed after the Pentagon discovered that the services had for years thrown out hundreds of millions of dollars worth of new or serviceable equipment while other units were purchasing identical equipment new.

"It was just a mistake," a senior Defense Department official said. "They miscoded the helmets as condemned, and it was during the freeze. We do make mistakes sometimes."

A report prepared in October by John W. Melchner, the Pentagon's assistant inspector general, says that auditors working for Inspector General Joseph H. Sherick heard of the helmet disposal during a more comprehensive investigation of the surplus-property problem.

The investigators happened on the problem when they found 141 helmets in good shape that had been turned in by the 2nd Armored Cavalry Regiment and labeled "H," for "unserviceable (condemned)." In fact, the auditors found that that batch of helmets, worth more than $2,300, should have been labeled "A" -- "serviceable (issuable without qualification)."

Digging deeper, the investigators learned that more serviceable helmets had been turned in to a second branch of the Defense Property Disposal Office in Seckenheim, West Germany, where they were "demilitarized" with an acetylene torch. The investigators said they were unable to learn "the quantity of demilitarized helmets, date of turn-in" or why they were scrapped.

The Army is replacing its time-honored steel pots with a new generation of helmets made from a plastic-like substance called Kelvar. The new helmets have been criticized for their resemblance to the World War II Nazi helmets, but the Army says the new helmets provide better protection and are more comfortable.

"Those steel pots that are in serviceable condition will go through the logistics mill and end up with reserve units that are short on steel pots right now," Maj. Don Maple, an Army spokesman, said.

Defense Department spokesman Jim Turner said that after investigators found good helmets in military junkyards, U.S. Army headquarters in Europe sent clarified turn-in procedures to all commanders and to 16 property-disposal units in Europe.

Lawrence J. Korb, assistant secretary of defense for manpower, installations and logistics, ordered the moratorium on disposal of surplus property last July. His order, which applied to all services and defense agencies, followed revelations in Congress that the Air Force had scrapped $700 million worth of spare parts in 1983, many of which could have been used.

The moratorium is still in effect for the Air Force, which has not come up with procedures that satisfy top defense officials, according to James H. Reay, deputy director for supply policy and programs. But the moratorium was lifted for the Army Oct. 10, he said