The full-page magazine advertisement by the National Rifle Association contains a riveting statistic about gun-control enforcement:

"Testimony by the Bureau of Alcohol, Tobacco and Firearms before numerous Senate hearings showed 75 percent of the cases were filed against law-abiding citizens."

The only problem with the statement is that it is demonstrably untrue. Officials at the Bureau of Alcohol, Tobacco and Firearms (ATF) said their records show the opposite. In the last three years, 74 percent of those recommended for prosecution on firearms charges had prior criminal records.

NRA spokesman Andrew Kendzie said that may be true today, but that the ad is based on 1979 testimony by "a former ATF official." He later identified the witness as Vernon D. Acree, a former Customs Service commissioner who never worked for ATF and was giving his estimate for cases in parts of Maryland and Virginia. Acree was a paid consultant to the NRA at the time.

The debate over the McClure-Volkmer bill, which would allow interstate sales of handguns and weaken the 1968 Gun Control Act, has been rife with charges of misinformation by both sides. The Senate has overwhelmingly passed the bill, which is backed by the Reagan administration, and House supporters need fewer than 20 more signatures on a discharge petition that would force the legislation to the floor.

Many proponents of McClure-Volkmer said that they have no quarrel with ATF's enforcement practices. Instead, they said that they are trying to prevent a recurrence of abuses that allegedly took place during the Carter administration.

Kendzie, defending the NRA's ad in Time magazine, said, "I don't think it gives the impression it's current information. It took us years to get even a modicum of these abuses exposed . . . . I don't think it's misleading at all."

But Acree, who oversaw ATF as an Internal Revenue Service official from 1958 to 1972, disagreed. "That wasn't testimony by ATF," Acree said. "I never worked for ATF. I'm a little concerned, frankly, that the NRA may have taken it a bit out of context . . . . That really skews the outcome." Acree also said he opposes much of the McClure-Volkmer bill.

Jack Killorin, chief spokesman for the Treasury Department firearms bureau, said the conviction rate for cases referred by ATF has consistently topped 90 percent. He said that 57 percent of the 7,963 potential defendants in the last three years had felony records, including 956 drug traffickers last year.

"We don't make technical record-keeping cases against anybody," an ATF official said. He said there have been only a handful of alleged "horror stories" in the 37,500 cases handled by ATF since 1970.

"The majority of dealers are simply law-abiding citizens, and we treat them exactly like what they are," the official said.

The NRA, for its part, said that opponents of McClure-Volkmer are waging a campaign of distortion against the bill. The NRA said that opponents wrongfully contend, for example, that the bill would weaken mandatory penalties for armed criminals.

On the other side, the nation's major law-enforcement groups are angry that NRA ads claim support for the bill from "national police organizations" and "rank-and-file police officers." The only major police group to publicly support the bill is the 17,000-member International Union of Police Associations.

The NRA's ad explains the dispute this way: "The leadership of the Fraternal Order of Police, International Association of Chiefs of Police and some others that claim to speak for their membership . . . have relied on distortions of the bill provided by antigun ownership groups, such as the National Coalition to Ban Handguns and Handgun Control Inc. The propaganda they hand out opposing the bill was written and printed by Handgun Control Inc. It is totally wrong and intended to mislead the public."

The NRA ad also says that the Gun Control Act has been "an enforcement nightmare" and that "honest citizens were turned into criminals for inadvertent technical violations of ambiguous gun regulations." At a recent House hearing, however, ATF Director Stephen E. Higgins said he has received few complaints about such practices.

Higgins was asked about provisions of the McClure-Volkmer bill that would bar his agency from conducting surprise inspections of gun dealers and limit such inspections to one a year. He said that the agency has been announcing its inspections in advance since 1982 -- except in criminal investigations -- and that it inspects licensed dealers an average of once every 25 years.

The NRA's Kendzie does not dispute such figures. "When the Reagan administration came to town, these guys cleaned up their act fast," he said.

In the past, Kendzie said, the NRA found numerous cases in which ATF agents improperly confiscated citizens' guns. He cited 1979 and 1980 Senate hearings in which several gun collectors and dealers testified that they were harassed by the ATF and that their weapons were seized for long periods, although no criminal charges were brought against them. ATF disputed some of the allegations

"These were primarily things that happened from 1977 to '80," Kendzie said. But he added, "If you've got the same bad law on the books and you don't change it, under another administration the same damn thing can happen . . . . If a Ted Kennedy administration came to town, you could go right back to the way it was three or four years ago."

At the ATF, officials believe the NRA is unfairly castigating the agency over old and disputed allegations. One official noted that the agency had been widely praised for respecting the civil rights of abortion clinic protesters while investigating a series of clinic bombings.

"We're not in the business of abusing people's rights," he said. "We don't have civil rights specialists for abortion clinic investigations, and growling neo-Nazi maniacs in the basement that we turn loose on gun dealers."