Rep. Tom McMillen (D-Md.), doing his best Mel Gibson imitation, crouched and fired 10 noisy rounds from a sleek black Beretta pistol in Beretta USA's plant in Prince George's County yesterday, signaling the start of the second round of intense competition to supply handguns to the military.

Beretta USA won the first round in 1985, snaring a five-year, $75 million contract to supply the Army with 316,000 of its new 9 mm 92F pistols. The contract sparked a major expansion of Beretta USA's plant in Accokeek, including the creation of 140 jobs and plans to hire an additional 100 workers.

But two Massachusetts congressmen, responding to complaints from Smith & Wesson of irregularities in the testing procedures, pushed through an amendment to a spending bill last year requiring the Army to conduct a new round of tests this summer that will cost an additional $15 million and will force the Army to start buying additional pistols that it doesn't expect to need until 1990.

The Army will issue invitations this month to Beretta USA, which had expected to handle all future gun orders, and other gun manufacturers to compete for a second contract to supply an additional 200,000 pistols, according to a spokesman. McMillen and Rep. Steny Hoyer (D-Md.), who both represent part of Prince George's County, say they will fight future efforts by House members to influence the Army's decision.

"Politics entered the fray once before and there's no way of knowing it won't enter again," McMillen said after completing an hour-long tour of the Beretta USA plant.

Robert L. Bonaventure, vice president and general manager of Beretta USA, said he is confident his company will win the second contract, but said the congressional requirment for additional testing puts a crimp in his long-term plans for the plant.

"To have things upset politically is very discouraging to us," he said.

A major battle among U.S. gun manufacturers was touched off nine years ago with the Army's decision to replace the venerable Colt .45 with a standard 9 mm handgun.

Army officials, piqued by the intervention of Reps. Edward P. Boland (D-Mass.) and Silvio O. Conte (R-Mass.) on behalf of the Bay State-based Smith & Wesson, say it could eventually result in two sets of 9 mm handguns, thereby defeating the Pentagon's original plan to provide all servicemen with one standard firearm. Testing for the second contract will begin in late October or November, with a final decision to be made early next year.

Maj. Philip Soucy, an Army spokesman, said yesterday that the military is delighted with the 80,000 Berettas already delivered.

"We're getting an excellent weapon at an excellent price and we're happy with it," Soucy said.

" . . . Smith & Wesson makes wonderful weapons. However, the weapons they sent us to be tested {in the first round} broke. This is not a minor failing."

Smith & Wesson officials, backed up by a General Accounting Office study, contended that the Army's original tests were biased against the Massachusetts firm because the firm's guns were fired in excess of the 5,000-round standard. But the Army contended that the company's weapon broke down after it was fired between 4,500 and 5,000 times.

"It's not that we are convinced we'll get the second contract," said Dennis Lee, Smith & Wesson's director of federal marketing. "It's that we know we were unfairly eliminated and not allowed to bid the first time."