Supporters of legislation pushed by the National Rifle Association to weaken the 1968 Gun Control Act won a key House showdown yesterday as they crushed attempts to continue the ban on interstate handgun sales and make other changes sought by police groups.

After six hours of debate, the House rejected a sweeping amendment offered by Rep. William J. Hughes (D-N.J.) to the McClure-Volkmer gun decontrol bill. The 248-to-176 vote was a stinging defeat for major law enforcement groups, which spent the day furiously lobbying for the changes, and provided fresh evidence of the NRA's influence on Capitol Hill.

Hughes later offered part of his amendment separately and was defeated by a similar margin. Further votes were postponed until today after House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), citing Gramm-Rudman-Hollings budget cuts, urged colleagues to avoid putting Capitol police on overtime.

Hughes said his amendment would "remove the label 'cop-killer legislation' that the police organizations have placed on the NRA bill." But the bill's chief sponsor, Rep. Harold L. Volkmer (D-Mo.), said the amendment would gut his measure, a version of which passed the Senate last summer, 79 to 15.

Gerald Vaughn, executive director of the International Association of Chiefs of Police, said he was surprised at the margin of defeat. "It would appear the House has ignored us on this issue," he said. "The NRA has a stranglehold on Washington . . . . It is a special-interest group dictating national gun policy. We'll have to deal with it on the streets of our cities."

Volkmer's bill, whose chief Senate sponsor is Sen. James A. McClure (R-Idaho), would lift restrictions on the transportation and sale of rifles and handguns; Hughes' amendment would have continued these restrictions for handguns. The amendment also would have required that local police be notified of all handgun sales, although it would have imposed no waiting period on the purchases.

The stage was set for yesterday's showdown when supporters of McClure-Volkmer garnered 218 signatures, a majority, on a discharge petition to force their bill to the floor.

Although the House Judiciary Committee had rushed out the Hughes compromise, the show of strength forced Hughes and his allies onto the defensive, and they adopted a strategy of amending Volkmer's bill rather than trying to pass their own.

During the daylong debate, Hughes called the Volkmer measure "a prosecutor's nightmare" that was "rife with loopholes for criminals" and "revolting" to police.

He said the bill would allow the sale of "untraceable weapons" and "make it next to impossible to convict dealers."

But Volkmer insisted that his bill "doesn't make it easier for felons to get handguns" and would "ensure that the civil rights of 200,000 licensed firearm dealers will not be abused."

He said Hughes' proposed definition of a gun dealer was so broad "that every gun owner has to be registered as a dealer."

In one hot exchange, Hughes asked Volkmer how anyone could oppose his ban on silencers, which he said have no sporting value.

"By banning silencers, you're not going to stop one crime," Volkmer replied.

The Hughes amendment also would have allowed two surprise federal inspections of each gun dealer annually, restricted unrecorded sales by dealers who transfer guns to their personal collections and tightened restrictions on machine-gun conversion kits. Unlike the Volkmer bill, it would not have required proof of "willful" violations to prosecute a gun dealer.

Hughes also sought to kill a Volkmer proposal that would override state and local restrictions on interstate transportation of firearms, saying that an armed criminal stopped in a car could simply claim he was heading out of state.

But Rep. John D. Dingell (D-Mich.), an NRA board member, countered that "the Hughes amendment and existing law make criminals every day out of law-abiding citizens" who go on hunting trips. "It doesn't stop the Mafia," Dingell said.

Dingell also dismissed Hughes' proposals as "an attempt to derail something which 218 members of the House have said is necessary."

At times the two sides sounded as if they were debating different bills.

Hughes and his supporters spoke of the 700 police officers killed in the last decade, the 1981 shooting of presidential press secretary James Brady and what Rep. Robert G. Torricelli (D-N.J.) called "a gun epidemic in America."

Volkmer's allies talked about the Second Amendment right to keep and bear arms, hunters' rights and alleged government harassment of gun dealers.

On the vote, 110 Democrats and 138 Republicans joined to defeat Hughes.

Locally, Reps. Michael D. Barnes (D-Md.), Steny H. Hoyer (D-Md.), Stanford E. Parris (R-Va.) and Frank R. Wolf (R-Va.) supported the Hughes amendment; Rep. Marjorie S. Holt (R-Md.) opposed it.