The disclosure is another embarassment for the FBI
A total of 184 computers are missing, including 13 that are believed to have been stolen. Three of the missing machines may contain classified information, said officials.
Out of 449 unaccounted-for firearms - including pistols, handguns, rifles, shotguns and sub-machine guns - 184 were stolen while 265 were lost, said officials.
The missing computers and weapons were discovered during a comprehensive inventory of equipment carried out at the request of the Department of Justice.
The disclosure comes hours before the opening of a Senate Judiciary Committee hearing into a series of high-profile FBI mistakes in recent months.
Blunders
In May, the bureau admitted that it had withheld evidence in the case against Oklahoma city bomber Timothy McVeigh, causing his execution to be delayed for nearly a month.
Months earlier, the FBI had been left red-faced when one of its former agents, Robert Hanssen, was arrested and later charged with spying for Russia over a 20-year period.
The federal government suffered another blow last year when the State Department admitted that it had "misplaced" a laptop containing highly classified information.
This followed accusations that the FBI had covered up aspects of the fateful siege in Waco, Texas in 1993 and arrested the wrong man in the bombing of the 1996 Atlanta Olympics.
In June, Louis Freeh stepped down as FBI director two years earlier than expected following severe criticism of the bureau after the Hanssen affair.
He has been suceeded by high-ranking Justice Department official Robert Mueller.
Equipment 'lost'
FBI officials said that the bureau has roughly 50,000 guns and 13,000 computers.
The FBI said it believes that 66 weapons were lost in connection with a retired agent, while about four were carried by agents who had either been sacked or had died, officials said.
Officials speculated that the laptops may have been lost as they are frequently passed around from office to office.
They also said they believed some weapons had were apparently been lost during training operations with other law enforcement agencies.
One official said a "small number" of the stolen weapons may have been used in local crimes, such as robberies.
Attorney-General John Ashcroft is expected shortly to confirm the missing equipment.