Gun-control advocates won significant modifications yesterday in a bill sponsored by Sen. James A. McClure (R-Idaho) and Rep. Harold L. Volkmer (D-Mo.) that would have eased federal restrictions on sale of handguns and other firearms.

A major change worked out behind the scenes would tighten language that might have jeopardized gun-control legislation in 20 states.

The agreement negotiated by the Senate Republican leadership is expected to lead to consideration of the measure after Congress returns from its Fourth of July recess.

Although the bill's foes remain opposed to many provisions of the legislation, including one that would allow purchase of handguns across state lines, they hailed the changes as a major improvement.

"It's a very good start," said Barbara Lautman, director of communications for Handgun Control Inc., a gun-control lobbying group. "It's not a victory, but it gets us in position for a good fight."

Gun-control advocates said the original legislation would have preempted state and local laws governing gun traffic within local jurisdictions. To meet a major objection of these advocates, Senate sources said, the legislation was tightened to allow interstate transport of unloaded, inaccessible weapons.

Gun-control advocates contended that the previous language -- ostensibly to facilitate travel of hunters and collectors -- was so broad that it would have nullified gun laws in 20 states, including Virginia and the District of Columbia.

A provision allowing gun dealers to operate at temporary locations was tightened to limit such sales to gun shows and other such legitimate events, the sources said. Critics had charged that the earlier language would have allowed gun sales from card tables on street corners.

Another provision to bar prosecution for "simple carelessness" was dropped from the legislation, although legislative-history language will be included in an attempt to prevent prosecutions for purely technical reasons, the sources said.

A final concession worked out yesterday at the insistence of Sen. Howard M. Metzenbaum (D-Ohio), who is allied with Sen. Edward M. Kennedy (D-Mass.) in leading the fight against the measure, would allow the government to ban importation of barrels, frames and receivers for guns not intended for sporting purposes.

The legislation represents a major change in the Gun Control Act of 1968.