Prince George's County Executive Parris Glendening, following the lead set by the Maryland State Police and D.C. police, is expected to announce tomorrow that the county's police force will convert its weapons to semiautomatic 9mm handguns made by Beretta USA Corp.

The upgraded sidearms are part of a national trend among police departments trying to catch up with the heavier firepower used by criminals, particularly those involved in the lucrative drug trade.

Although county officials refused to confirm that the 915-member police force would soon switch to the 34-ounce, 16-shot semiautomatic weapons, Warren Barron, Beretta's sales manager for law enforcement and training, said, "We will be meeting with Prince George's County on Monday to finalize how many units they will need" and other details of the deal.

The new guns, whose semiautomatic mechanism allows for more rapid firing, would replace a heavier .38-caliber Smith & Wesson revolver that holds six shots.

The county is expected to buy 1,200 of the Beretta 92F models for about $300 each, and the total cost of the deal will be about $500,000, including bullets, clips and training.

The new handguns would give officers an additional 10 bullets before having to reload, and reloading is faster, experts say, because it involves merely attaching a clip filled with bullets to the gun.

Unlike the service revolvers now in use, the Beretta also has an external safety that permits the user to lock the firing pin, preventing accidental discharge and giving the officer time to recover the gun if it is taken from him during a scuffle.

Beretta, a subsidiary of the gun company owned by the same Italian family for 456 years, employs 450 full-time workers at a plant in Accokeek in Prince George's County. "We are in a full production capacity and the first 300 guns could be shipped within 30 to 90 days," Barron said. Training would begin immediately at the county police academy in Forestville.

Recent events, such as the drug-related shooting deaths of five people in Landover on Jan. 23, have prompted increased attention on firearms. "The drug dealers are now very well-armed and the police officers should at least have comparable weapons when dealing with issues of this magnitude," said State's Attorney Alex Williams.

As drug-related violence increases, police throughout the area have become alarmed at the prevalence of high-powered weapons. District police officials said last week that thay have confiscated 50 percent more weapons this year than in 1987. On Thursday D.C. police discovered 13 high-powered weapons -- including a machine gun, a four-barrel .357 magnum, two 9mm semiautomatic handguns and a 10mm semiautomatic handgun -- in the trunk of a Lincoln Continental occupied by two Virginia men. They were stopped at 14th and L streets NW after a prostitute alleged that the men had aimed a gun at her.

Although Prince George's police could not provide figures on the numbers of recently confiscated guns, one police official said, "We are having the same problem {as the District}, only on a smaller scale." Although the official acknowledged that police seldom have been the target of aggression by well-armed drug dealers, he said the county's switch to semiautomatic handguns would make police a "better match" when the occasion arose.

Col. Elmer H. Tippett, the newly appointed Maryland State Police superintendent, cited the gun's increased accuracy, expanded capacity and ease of reloading when he announced last month that state troopers would soon be armed with the Beretta semiautomatic. Two weeks ago, D.C. Police Chief Maurice T. Turner Jr. announced that the District's 3,880-member force also would be equipped soon with 9mm semiautomatic handguns, although a brand has not been selected.

Barron said Beretta has also agreed to provide semiautomatic weapons for Charles and Anne Arundel counties, the Connecticut State Police, and the highway patrols in North Carolina, Wyoming, and South Dakota. The company, which in 1985 was awarded a $75 million contract to supply the Army with 316,000 9mm 92Fs, has received inquiries from police departments in Montgomery, Howard, Calvert and Baltimore counties and Baltimore City, according to Barron.