At best, the four-year-old U.S. Postal Service Management Training Center has been a tolerated neighbor in the affluent suburb of Potomac.

Every year, upwards of 12,000 postal managers from around the country attend the Potomac facility, taking one- to three-week courses on such subjects as general management, safety and basic computer skills.

In 1980, residents had sued unsucessfully to block the Postal Service's purchase of the 84-acre tract on Kentsdale Drive from the Sisters of Mercy. The postal facility, one of the largest training institutes in the area, would generate too much traffic, they said, and would disrupt the neighborhood, McAuley Park, a subdivision of 100 families just east of Potomac Village.

Now, residents are upset again about the Postal Service's plan to increase parking and build an indoor firing range at the facility. The range would be used by postal inspectors, who as sworn law enforcement agents are required to carry weapons. They come to the institute for 11 weeks of training. The plan represents a serious breach of the agency's original concessions to the community, the residents said, and raises concern about further expansion.

"This is a case where democracy is absolutely stymied," said Potomac resident Fred Daley, an attorney and spokesman for a citizens' group. "An agency like the Postal Service can walk into a neighborhood and do whatever they want. They don't answer to anybody and there's not a thing we can do about it."

Last week, the National Capital Planning Commission approved plans to build four tennis and two handball courts on the property and to make other technical improvements, but withheld approval of an additional 174 parking spaces and a state-of-the-art indoor firing range. The planning commission urged the postal service to discuss the project further with the Montgomery County Planning Board, which had already formally expressed its dissatisfaction.

"The situation is viewed as an erosion of the good will that had been established between the Postal Service and the community," said Thomas Robertson of the county planning board, the only speaker before the National Capital Planning Commission last week.

"Originally in 1980 there was a promise made by postal authorities that students would not be driving their cars in any great numbers," he said. "That promise was made at a time when gasoline was expensive and hard to get, so it was an easy promise to make and keep. But what they're planning now is not in keeping with the original agreement."

The parking expansion is resented because of the automatic increase in traffic in an area almost exclusively residential, he said.

"The firing range," said Robertson, "would also be an attraction to people seeking guns. You hear about guns stolen from federal arsenals from time to time. It happens. Also, firing ranges like this are rare and other agencies would probably end up using it. There again, you would have an increase in traffic."

Postal Service spokeswoman Meg Harris said residents need not fear that the agency will significantly expand its operation in Potomac.

"Our aim is not to extend our labor force," Harris said, "so it follows that we would not be expanding the training facility for our labor force. What we have in Potomac now is pretty much what we're going to have."