Six shots were fired aboard Pacific Southwest Airlines Flight 1771, at least four of them inside the cockpit, shortly before the jet crashed Dec. 7, the FBI said yesterday.

The cockpit voice recorder reveals "three sharp reports" inside the cockpit, followed by sounds of commotion and a fourth sharp report soon before the tape ended, Richard T. Bretzing, special agent in charge of the Los Angeles FBI office, said in a statement.

The details provide a clearer picture of the final moments before the four-engine, British-built BAe-146 plummeted nose first into a hillside in the coastal range northwest of San Luis Obispo, killing all 43 persons on board.

But the FBI refused to speculate whether David A. Burke, a fired USAir employe who agents believe smuggled a handgun aboard, shot the two pilots and himself shortly before the crash.

A .44-caliber Magnum and six expended shells were found at the crash site, along with an unsigned note addressed to "Ray."

Investigators have said they believe that the note was intended for Ray Thomson, the airline supervisor who fired Burke Nov. 9 after Burke allegedly stole $69 worth of liquor receipts. The FBI traced the gun to a friend of Burke's in San Francisco, who said he loaned it to Burke this fall.

On the cockpit recorder, two noises that sound like gunshots are heard just before the pilot tells air-traffic controllers at the Oakland Center that the pilots had heard gunfire.

After declaring an emergency, one pilot is heard saying that he is taking the plane to a lower altitude.

A female voice utters a one-word warning to the captain, followed immediately by sounds of forced entry into the cockpit. The FBI declined to confirm whether the female voice was that of a flight attendant and refused to reveal the word she uttered. The female was described by the FBI as speaking in "a controlled voice."

Immediately after the entry, the three sharp reports are heard, followed by sounds of the commotion. The final sharp report is heard just before the tape ends.

Jim Kneelson, an FBI spokesman in Los Angeles, declined to say whether the four shots sound different from the two shots reported earlier by the pilot. Kneelson also declined to discuss other details on the tape.