Opponents of the McClure-Volkmer gun decontrol bill, in a major shift in tactics, plan to offer a flurry of amendments to the measure today as the House begins its first major debate in 17 years on changing the 1968 Gun Control Act.

The opponents had been hoping to pass compromise legislation sponsored by Reps. William J. Hughes (D-N.J.) and Peter W. Rodino Jr. (D-N.J.), which sailed through the Judiciary Committee 35 to 0 before the Easter recess. But Hughes now thinks that he lacks the votes for his bill and has drafted a sweeping amendment that reflects the concerns of the nation's law enforcement groups.

But supporters of McClure-Volkmer, led by the National Rifle Association, are optimistic about passing their version intact. They persuaded 218 House members, a majority, to sign a discharge petition last month to force the bill to the floor. This show of strength led to adoption of wide-open procedures that will allow floor amendments to both bills.

Chief House sponsor Harold L. Volkmer (D-Mo.) recently made minor changes in his bill, which among other things would lift a ban on interstate sales of handguns, ease record-keeping rules for gun dealers and impose mandatory sentences for using a gun in a violent crime. But leaders of a coalition of police groups still regard the measure, which passed the Senate overwhelmingly, as unacceptable.

"A lot of members may vote for the Volkmer substitute, no matter how tightly they have to hold their nose," a Hughes aide said. To win votes from NRA supporters who are also sympathetic to police concerns, he said, Hughes' amendment will be "cast as the law enforcement vote."

The amendment would continue the ban on interstate sales of handguns, while allowing sales of rifles and shotguns across state lines. It also would ban silencers, continue to restrict ammunition sales to licensed dealers and allow two surprise federal inspections of each gun dealer each year.

"The cops consider this bare minimum, bottom-line stuff," said Police Foundation spokesman Tom Brady.

But Gary Curran of Gun Owners of America said his group is opposing all amendments to the Volkmer bill as "various forms of gun control." NRA spokesman Wayne LaPierre said the revised Volkmer bill is "a moderate piece of legislation."

Other planned amendments should prove equally controversial. Hughes and two other House members may propose a mandatory waiting period of up to 15 days to allow local police to check the backgrounds of persons wishing to purchase handguns. Hughes also may offer an alternate approach that would impose no waiting period but would require that local police be notified of handgun sales.

In addition, Rep. Sala Burton (D-Calif.) may propose to ban the manufacture and sale of cheap handguns known as "Saturday Night Specials," while Rep. Robert J. Mrazek (D-N.Y.) may try to ban new plastic handguns that are not identified by metal detectors. At the other end of the spectrum, Rep. Larry E. Craig (R-Idaho) plans an amendment that would allow mail-order sales of rifles and shotguns.

Police leaders say the Volkmer bill would make it easier for criminals to obtain handguns and harder for police to trace illegal firearms. But NRA officials argue that criminals obtain their weapons on the black market and that local laws requiring police notification have had little effect on crime.

Sarah Brady, wife of presidential press secretary James S. Brady, has been lobbying against the Volkmer bill on behalf of Handgun Control Inc. Her husband was seriously wounded in the 1981 assassination attempt on President Reagan.

The measure's chief Senate sponsor was James A. McClure (R-Idaho).