The Senate approved and sent to President Reagan yesterday a House-passed bill that weakens the 1968 Gun Control Act, but also passed several strengthening amendments as part of a compromise with law-enforcement groups.

The outcome gave the National Rifle Association and the bill's main sponsors, Sen. James A. McClure (R-Idaho) and Rep. Harold L. Volkmer (D-Mo.), most of what they wanted in their 18-year effort to ease federal restrictions on gun owners and gun dealers.

At the same time, a compromise engineered by Senate Judiciary Committee Chairman Strom Thurmond (R-S.C.) narrowed some of what the nation's police groups regarded as dangerous loopholes in the measure. The Senate passed both bills -- the McClure-Volkmer measure and Thurmond's clarifying amendments -- by voice vote.

After backstage negotiations that tied up the Senate for hours, it was agreed that McClure, Volkmer and the 3-million-member NRA would not oppose the strengthening amendments in the House.

The Thurmond provisions require gun dealers to keep records of sales from their personal collections, without which police say it would be impossible to trace many guns used by criminals. It also clarifies the definition of a gun dealer and specifies the extent to which people carrying guns across state lines must keep them unloaded and inaccessible.

In striking the deal, the bill's proponents avoided a long series of amendments -- to be offered by Sens. Edward M. Kennedy (D-Mass.), Howard M. Metzenbaum (D-Ohio), Charles McC. Mathias Jr. (R-Md.) and others -- on such proposals as banning silencers and plastic handguns. Liberal senators believed it would have been difficult for Republican lawmakers seeking reelection this year to vote against these amemdments.

The Reagan administration supported McClure-Volkmer and the president is expected to sign it.

Police and gun-control groups were pleased to win changes in the bill backed by the NRA, which trounced House opponents last month in a victory widely attributed to the group's political clout and its high level of campaign donations to members of Congress.

In passing its version of the gun decontrol bill, 292 to 130, the House eased federal restrictions on interstate transportation of handguns and rifles. At the urging of police groups, however, the House dropped a provision that would have lifted a ban on interstate sales of handguns, and added a ban on sale of machine gun parts.

McClure preferred his original measure, which passed the Senate last summer and included eased restrictions on interstate handgun sales. But McClure said he accepted the House version to avoid a divisive House-Senate conference that could have killed the legislation for the year.

Asked about the Thurmond amendments, McClure said, "I'm not very enthusiastic about the bill because I think it's unnecessary and most of its provisions are covered in my bill." He said he preferred total repeal of the 1968 Gun Control Act.

The bill going to Reagan restricts federal inspections of gun dealers, eases record-keeping rules for dealers, requires evidence of "willful" violations in prosecuting dealers and mandates minimum sentences for those using a gun in a violent crime.