The Justice Department, reversing its previous tentative support for legislation banning hard-to-detect handguns, has given its "enthusiastic" endorsement to a compromise measure, backed by the National Rifle Association, that would outlaw only guns made entirely of plastic.

Attorney General Edwin Meese III, in a letter to Sen. James A. McClure (R-Idaho), the chief Senate sponsor of the NRA-backed bill, termed the measure "an excellent legislative proposal to deal with the threat posed by criminals and terrorists armed with such weapons.

"I am pleased to advise you that you have the enthusiastic support of the Department of Justice" for the legislation, Meese said.

The McClure bill -- which represents a turnabout from the NRA's previous position that no legislation addressing plastic guns is needed -- would make plastic guns unlawful unless their barrels are constructed at least partially of a metallic substance capable of being detected by a magnetometer or the serial number required by federal law is engraved in metal. In addition, the bill requires that the plastic part of the gun be "infused" with sufficient quantities of a compound to make it show up on an X-ray machine.

Police organizations and gun control groups assert that the McClure bill would offer inadequate protection because it would permit the manufacture of guns that contain only tiny amounts of metal that could pass through metal detectors set at conventional levels. Instead, they are pushing legislation that would ban guns containing less than 8 1/2 ounces of metal.

Yesterday, they assailed Meese's action. "We're extremely disappointed," said Jerald R. Vaughn, executive director of the International Association of Chiefs of Police. "It's another example of how this administration constantly capitulates to and panders to the gun lobby. . . . We're quite frankly sick and tired of hearing this administration tout their support for law enforcement because it's just not there."

At a meeting with police officials last year, Vaughn said, "the attorney general held in his hand small weapons that he proclaimed he could see absolutely no legitimate use for and that he agreed that some type of legislation needed to be enacted that would ensure that nondetectable weapons be effectively dealt with. Apparently, he now thinks that those kinds of weapons are okay."

Justice Department officials earlier approved a package of gun control legislation that included such a provision, and sent it to the Office of Management and Budget for review.

But after an intense lobbying effort by the politically potent NRA, Meese had the bill "pulled back" from OMB in October, Justice Department spokesman Patrick S. Korten said at the time.

Korten said yesterday that Meese had not caved in to NRA pressure. "It's not a matter of knuckling under at all," he said. "It's a matter of achieving a consensus here that I think if they all look closely they will find meets the needs of law enforcement as well as addresses the concerns of legitimate, mainstream gun owners' groups."

Vice President Bush brought the issue into focus last month in New Hampshire at a gun owners' forum for the presidential candidates. Bush pulled a 5 1/2-ounce, five-shot, .22-cal. pistol from his suit pocket and, while vowing to oppose legislation that would allow plastic guns, urged the gun owners to "sit down" and work out a compromise with police groups.

Joseph A. Morris, director of the Justice Department's Office of Liaison Services, said police groups are not taking into account technological developments in detection equipment. "We have learned within the last week that the technology has evolved to the point where even the smallest of existing legitimately manufactured handguns cannot only be detected, but its presence can be distinguished from the presence of other metal-containing objects such as key, watches or calculators with an increasingly practical degree of certainty," he said.

But Vaughn and other opponents of the bill charged that airport security devices would have to be set at such a sensitive level to detect the guns that passengers would be lined up for hours.

"It's a callous and arrogant disregard for the rights of millions of travelers who will have to endure this inconvenience so that this administration can take good care of their friends in the National Rifle Association and other progun groups," Vaughn said.

James Jay Baker, the NRA's di rector of governmental affairs, said the bill addresses the problem of airport security without banning existing guns. "I believe that the American public is willing to wait a little bit longer, because they're waiting at airports for a lot of reasons that aren't as compelling as airport security," he said.