Hunting rifles are displayed for sale at Firearms Unknown, a gun store in Oceanside, California, U.S., April 12, 2021. Reuters / BING GUAN

U.S. Senate negotiators, racing to settle details of bipartisan gun legislation, on Wednesday struggled to resolve serious disagreements over federal funding of state "red flag" programs and the breadth of a plan for keeping guns out of the hands of those prone to domestic violence.

A string of mass shootings has prompted Democratic and Republican lawmakers to line up behind gun-violence legislation that would overcome decades of inaction.

Lawmakers hoped to get it passed as soon as next week, before the start of a July 4 holiday recess.

But Senator John Cornyn, the main Republican negotiator on the bill, said that might not be possible.

"If we continue down this path without resolution, then we're jeopardizing the timetable ... or we're jeopardizing the likelihood that we can get to 60 votes for anything."

Sixty votes are needed in the 100-member Senate, where control is evenly split between the two parties, to move most bills toward passage.

However, Democratic Senator Kyrsten Sinema, another lawmaker involved in the effort, said they were not running into significant problems. "We're going to get this all done. Things are good," she told reporters.

Cornyn has raised concerns over a provision to fund state "red flag" laws allowing authorities to temporarily take guns away from people deemed dangerous to themselves or others.

Cornyn's home state of Texas does not have such a law and is seen as unlikely to enact one. He wants the federal funding to cover other efforts as well, such as outpatient "crisis intervention programs" for people suffering from mental illness.

Senators and aides have not said how much federal funding is at stake.

Lawmakers are also at odds over an effort to close the so-called "boyfriend loophole," which allows authorities to block abusive spouses from buying firearms - but does not cover people who are not married.

Christian Heyne, a vice president at the Brady gun control organization, called that "a loophole that defies logic."

Lawmakers have been unable to settle on who should be included in the provision, such as two people who only had one date, for example, or only those in a longer-duration relationship.

Despite these differences, the lead Democratic negotiator, Senator Chris Murphy, told reporters that lawmakers remain committed to translating their framework deal into a bill that can be passed by Congress and signed into law by Democratic President Joe Biden. "I have confidence we'll get there," Murphy said.