As dart boards go, it is tattered and unstable. It consists of a RIF memo taped to a round board, and rests precariously on the wire door to the room where illegal explosives and firecrackers are kept.

It is, however, an uncommonly well-suited target for agents of the Treasury Department's Bureau of Alcohol, Tobacco and Firearms. Behind that wire door they keep samples of illegal fireworks bearing militaristic-sounding names such as M80s, blockbusters, silver salutes and cherry bombs, because one of their jobs is to see that homemade July 4 celebrations consist of pops and whooshes but no loud bangs capable of blowing off the merrymaker's hand.

The bureau does not resound with an air of festivity, and some agents say fireworks investigations, like much of the bureau's activity, have slowed this spring and summer because of a budget crunch and uncertainty about the future.

"When your head's in the guillotine and somebody is slowly cutting all the threads, it's difficult to concentrate on your work," said Warren L. (Roy) Parker, explosives enforcement officer.

The Reagan administration first proposed eliminating the bureau and assigning responsibility for federal enforcement of firearms and explosives laws to the Secret Service.

When gun advocates, who had lobbied for the bureau's elimination, contemplated Secret Service enforcement instead, the bureau found a reprieve.

But the future is uncertain. The budget shrunk to $115 million this fiscal year, compared with $150 million last year. An additional $23 million is tied up the congressional hassle over the "urgent" supplemental appropriations bill. The number of employes in the bureau has decreased in one year from 3,400 to 2,700.

"Being a lower priority than bombings, which involve murders and attempted murders, we probably have worked fewer illegal fireworks cases this springtime," Parker said.

Robert F. Dexter, chief of the explosives technology branch, agreed, citing figures showing that in recent months the bureau has investigated 40 percent fewer explosives cases (including illegal fireworks) than it did during the same period last year. No comparison of fireworks-only cases is available.

But Robert J. Creighton, head of the explosives enforcement branch, insisted that fireworks investigations have not slowed. "We do not work investigations based on the amount of money we have," he said.

The bureau is concerned only with so-called "Class B devices," a category assigned larger, more dangerous fireworks, which are are seen legally only in community fireworks displays. Manufacturers need federal permits to make Class B devices. The Consumer Product Safety Commission and many state laws regulate less dangerous fireworks.

Virginia and the District of Columbia permit small fireworks, while Maryland prohibits all fireworks except sparklers. Montgomery County goes further and bans even sparklers.

Agents inspect legal factories to ensure that fireworks meet safety standards and are stored carefully, and try to uncover illegal factories and distribution rings.

Producing illegal fireworks, which have an explosive force up to several hundred times the legal limit, can be lucrative. Dexter said an illegal factory can make 10,000 M80s in a day for 5 cents apiece and sell them for $1 each.

It is dangerous, however. Safety standards tend to be lax, and every now and then such a factory blows up. One exploded in Newport, Ky., last October, demolishing a large area.

Despite precautions, legal fireworks factories sometimes blow up, too. The Buckeye Fireworks Co., near Youngstown, Ohio, exploded three weeks ago, injuring three persons. And three persons were killed in an explosion at the Melrose Display Fireworks Co. in Orland Park, Ill., in 1972.

Playing with illegal fireworks also can be dangerous, particularly because they may be faulty. In a one-week period around Christmas, 1979, 130 persons injured themselves in a four-state area in the South while setting off illegal M80s with fuses that burned too quickly. Fourteen persons had fingers amputated and several others had injured eardrums.

The Consumer Product Safety Commission, which oversees legal, Class C fireworks and helps the bureau shut down illegal producers, estimates that 40 percent of fireworks injuries result from illegal fireworks, even though they are only a fraction of the total used. The commission in 1973 unsuccessfully proposed a federal ban on all fireworks.

Paul J. Galvydis, a CPSC compliance officer, said one problem device this year was a legal contraption consisting of seven mortar tubes, each of which could send a fireball 50 feet into the air.

The device frequently fell on its side after the first or second tube went off so that the remaining fireballs went scooting along the ground. The agency's technician found himself dodging fireballs in the laboratory when the device fell over during testing. The firework, made in China, like many others, was withdrawn from stores.

There were a record number of fireworks injuries last year, about 11,400. The previous peak, 11,100, came during the Bicentennial in 1976. CPSC figures show that the number of injuries, as well as the amount of fireworks used, has crept up steadily in the last decade.