Robert Lee Miller Jr., the flamboyant contractor-horseman whose murder brought to light a federal investigation of alleged labor-business rackeeering, "just wasn't a man of moderation," a friend said yesterday.

When Miller visited the New Carrollton Metro station, where his Interstate Bridge Co. has a $16 million contract, he often rode in his six-door Mercedez-Benz, accompanied by a statuesque secretary dressed in western garb that matched his own.

When he went to Pimlico Race Track in Baltimore to bet on the horses he owned, he once arrived in a helicopter. When business took him on short notice from his home base in Frederick County, he might charter an airplane at the local municipal airport.

Wherever he went, Miller usually carried a wad of big bills and a loaded revolver. When police found his body Tuesday night, there were $1,900 and his unused loaded revolver in his cowboy jeans.

Uncharacteristically, however, Miller initially went to the room in the Ramada Inn without his gun, according to sources. He returned to his waiting limousine a few minutes later, apparently sensing trouble at the pre-arranged meeting, and got his gun and a bottle of whiskey, according to those same sources.

Sometime between his arrival at 6:30 p.m. and 8:30 p.m., when a wary room clerk used a passkey to enter the room Miller was killed. All the while, his chauffeur, Carlton Griffith, waited in the motel parking lot in Miller's silver Continental, which is equipped with a refrigerator, telephone, television and stereo and adorned with a ram's head on the hood.

Montgomery County homicide detectives would not comment on whether they had any leads or a motive in the killing. The U.S. attorney's office in Baltimore had subpoenaed the 44-year-old Miller as a witness in its investigation of labor-management racketeering in the construction industry, but local investigators would not say whether they believe Miller's death is related to that investigation.

"Cowboy Bob," as some called Miller, is to be buried today, in a blue leisure suit and flowery western shirt in his native Hartford County.

Miller grew up in Harve de Grace, in northeastern Maryland, the only child of a state highway department foreman. In the early 1950s, the young Miller also got a job with the highway administration, working in its bridge design division.

About 20 years ago, he left there to work for Kline Co., a Frederick County construction firm, and became head of its bridge division. Within a few years, the hard-working, ambitious and became owner of his own firm, which now regularly gets multi-million dollar contracts in a several state area.

As his contracts increased, Miller's horizons widened. He purchased a one-third interest in a racehorse, and eventually became the owner of one of the larger thoroughbred stables in Maryland.

Along the way, Miller developed a reputation as a big spender, betting heavily at the tracks where his horses ran, and spending lavishly on his men and women companions.

"A guy who moves that fast is bound to pick up some enemies," said one friend.

In New Market, the tiny Frederick County village where Miller's contracting operations were headquarters, Mayor Franklin Shaw said that if there was little mourning for Miller, it was because "it's easy to react negatively to a guy who drove down the street in a big limousine with his chauffeur. He wasn't the tpye you'd meet once and say he was a nice, friendly, guy."

Across the county near the Potomac River, in the even smaller village of Petersville, where Miller lived with his wife until their separation a few months ago, the mood was more sympathetic.

"Those Millers are nice people,"said Anna Terry, a waitress at the Crossroads Inn, where Miller, his wife, Margaret, and their children. Kenneh W., 24, and Deborah, 21, were regular customers.

Pat Main, another waitress, said,"all the kids around here swam and fished" in the pond on Miller's 146-acre farm. The Millers have lived about 20 years in a yelow frame and stone restored 19th century farmhouse that overlooks the pond and has an unobstructed view of the West Virginia hills across the river.

In addition to the limousines, which normally were kept at New Market, Miller owned two Corvettes and assorted other vehicles which were based at the farm for use of his family.

Margaret Tibbs, who lives in a modest frame house at the end of the dirt lane that leads to Miller's house, said that in the eight years she has lived there "we never spoke. He'd ride up and down this road with his chauffeur like he owned the world and all that's in it."

Petersville residents said all of the Millers wore western clothes, including the chauffeur.

"Everyone makes a big deal about the way dad dressed," complained Kenneth Miller. "He just liked western clothes. What's wrong with that?"

Sandy Kaufman, who is the secretary to David Weinberg, Miller's lawyer, friend and former partner in a racing stable, said Miller began affecting cowboy garb after his first trip to the west some years ago.

"The cowboy clothes and boots never bothered me," said Miss Kauffman, who works in a law office that has its own unusual decor, including a dozen paintings of nude women on the walls.

Miss Kauffman, who was watching a soap opera on television, paused and observed, "I guess watching these things is appropriate. This whole thing's been like a soap opera.But that sort of fits Bob. Everything he did was dramatic, one way or the other."