The big showdown was a mismatch.

The Washington Bullets didn't let Atlanta in the game, methodically dismantling the Hawks, 100-78, before 14,281 at Capital Centre last night and moving to within a half-game of both Atlanta and New York in the race for the last two playoff spots in the Eastern Conference.

The victory was as dominating, against a good team, as any this season. The Hawks led only once, at 2-0. The Bullets, who shot 59 percent, limited the Hawks to 37 percent shooting. They also outrebounded Atlanta, 44-35, and held the visitors to their lowest scoring total of the season.

The Bullets had outstanding performances from two former Hawks, reserves Don Collins and Joe Kopicki. Collins made seven of 11 shots and had three assists and four steals while Kopicki, now the backup power forward, had 11 rebounds and six points.

Atlanta (41-38) and New York (40-37) are tied for the fifth-best record in the conference, but the Bullets (40-38) have won 11 of their last 13 games and the Knicks and Hawks are hearing footsteps.

"We went into the game not wanting to just beat them (the Hawks), but to make it obvious that we were the better team," said Frank Johnson.

"I think we did it."

However, the Bullets lost the season series to both Atlanta and New York, so they must finish a game ahead of one of them to make the playoffs.

"This is the best position we've been in a month," said Bullets Coach Gene Shue. "This was the game we had to get, no matter what went on in the meantime."

The Knicks, idle last night, play at Philadelphia today. The Bullets and Hawks have the next two days off.

The only reason the Bullets didn't put away the Hawks sooner was because Dan Roundfield scored 23 of his 28 points in the first half, which kept the Hawks within 50-43, at halftime.

However, the Bullets had an incredible third period. They made 14 of 18 shots (78 percent) and outscored the Hawks, 30-16.

That enabled them to take an 80-59 lead into the final quarter and they expanded it to as many as 25 points.

Collins scored eight points in a three-minute stretch late in the third period and his fast break layup gave the Bullets a 75-59 lead with 22 seconds left in the period. And then the Bullets, remarkably, scored five points in the last one second of the quarter.

Johnson stole the ball from Roundfield and passed up court to Collins, who flipped it back over his head to Greg Ballard for a layup, as he was being fouled by Johnny Davis with one second left.

Ballard made the free throw and then Roundfield threw the inbounds pass away without it touching anyone, which gave possession to the Bullets under the basket. Ballard inbounded to Johnson, who made a layup at the buzzer.

Jeff Ruland got into early foul trouble, but still led the Bullets with 24 points. Ballard scored 22 points.

But it was the play of Kopicki and Collins that Shue wanted to talk about most.

"Collins had a sensational night and Kopicki was a key," said Shue. "When Jeff got into foul trouble, Joe came in and really helped."

Ruland picked up his third foul with nine minutes left in the half and Kopicki played the rest of it.

"We did a lot of tricky things tonight," said Shue. "We trapped a lot and forced the Hawks to do things they didn't want to do."

The Hawks had won three of their previous four games and beat Chicago, 138-101, Friday when they shot 64 percent and had 47 assists.

But the Bullets, too, were coming off a big victory, 107-99, at Boston, and they looked more prepared to play than the Hawks.

Besides Roundfield, the only other player in double figures for Atlanta was Davis, who had 13 points and 12 assists.

Atlanta was never able to establish a flow to its offense and in the first half went almost exclusively to Roundfield. He made 10 of 15 shots in the first half, but only two of seven after intermission. Rick Mahorn was on him most of the game.

"The only adjustment he made was that he played him better in the second half," said Shue.

"It's pretty tough for a player to play two halves like Roundfield played the first, especially against our defense."

A team of Washington Redskins defeated a squad of former Bullets, 24-22, in a 20-minute preliminary game last night.

Tony Peters had seven points and Todd Liebenstein scored six for the Redskins. Freddy Carter and Jack Marin had five apiece for the Bullets.

The game was not an artistic success: the Bullets shot 24 percent and the Redskins 28 percent. Redskins receiver Charlie Brown missed nine of 10 shots. Bullets Coach Gene Shue touched the ball twice and turned it over twice.