The Washington Bullets beat the Houston Rockets tonight, 123-115, and defeated an assumption at the same time.

When the Bullets embarked on their two-week, six-game road trip to the Pacific Northwest and Texas, it seemed to be a safe assumption that if the team were going to be outmatched in any of the games, it would be in the finale against the Rockets.

The Rockets, with their starting front line of 6-foot-9 Rodney McCray, 7-foot Akeem Olajuwon and 7-4 Ralph Sampson, seemed too overpowering for Washington.

So much for assumptions.

Leading by 21 points at various points in the second half, the Bullets registered a fairly easy victory at the Summit to end their extended tour with a 2-4 record. Gus Williams, in a shooting slump at the start of the trip, finished it with a bang, going 12 for 21 from the field and scoring 30 points, two more than fellow guard Jeff Malone.

Forward Greg Ballard scored 20 points for the Bullets, many of his baskets contributing to Williams' total of 11 assists. For the game Washington shot 60 percent, compared to 49 for Houston, which was led by Olajuwon with 29 points.

Sampson, weakened by flu, scored 14 points and had 12 rebounds. Yet he and Olajuwon really weren't a factor in the game, which didn't seem to surprise Washington Coach Gene Shue.

"I never thought much about the idea of our being outmatched," he said. "We played them in an exhibition game this fall and we didn't have much trouble with them then."

Shue added that the Bullets also lost that preseason encounter. A similar fate appeared probable tonight when the Rockets jumped out to a quick 8-2 lead, the first three baskets being scored by Olajuwon.

Tom McMillen, who was guarding Olajuwon at the time, was quickly replaced by Cliff Robinson. At the same time, the Bullets began to move, with Williams and Malone in the forefront. After one quarter, Washington held a 28-26 lead.

That margin jumped to 11 points in the first six minutes of the second period, the Bullets going on an 18-9 run to take a 46-35 lead. Keying the run were Dudley Bradley and Charles Jones, each scoring six points.

Ahead at the half, 64-53, Washington continued to surprise in the third quarter, outscoring the Rockets, 21-11, in the opening eight minutes to go up by 21 for the first time, 85-64.

It was at that point that Houston Coach Bill Fitch inserted guard John Lucas into the lineup. Lucas, who was reinstated this week following a 70-day supension for drug abuse problems, got a warm reception from the Houston fans that also seemed to perk up the team.

The Bullets remained in control of the game, only showing signs of coming apart with about 8 1/2 minutes to play.

Houston, which scored four three-point field goals in the final quarter, got two from Allen Leavell, the second completing an 11-6 run and making the score 113-104, Washington, with 3:53 remaining.

When Leavell hit another long shot and Sampson scored on a hook, the Bullets' margin was down to 120-115 with 1:07 left. But Williams hit a free throw and Malone added another two to put the game out of reach, the Rockets being unable to add to their final total.

"This has been a long trip and playing back to back like we did (Washington lost, 110-101, in Dallas on Friday), I thought we played a terrific game," said Shue. "Our shooting percentage, which hasn't been good during the trip, was there tonight. That was mainly because of Gus. Tonight he was terrific and that made a big difference in the game."

Perhaps the most surprising statistic was the rebound total, Washington finishing with 36 (11 by Robinson, nine by Rick Mahorn) to Houston's 32.

That margin was caused in part by the Bullets' shooting. Houston didn't get many rebound opportunities, which limited its fast break.

"I told the guys after the game, that's as good as anyone has played against us all season offensively," said Fitch. "I don't think we will see anybody shoot it any better against us the rest of the season. Someone may shoot 75 percent but they would have to be layups."

There weren't many of them scored by the Bullets tonight, but given the Rockets' overall height, that wasn't surprising. What was surprising was how few were needed.