Democratic Senator Dianne Feinstein said her proposal would be left out of the firearms control bill.
Democratic Senate leader Harry Reid made the decision, saying the proposal could not get enough votes.
An assault-type weapon was used in the December massacre that killed 26 at Sandy Hook primary school in Newtown, Connecticut.
The shooting shocked the US and revived efforts in Washington DC to prohibit such firearms.
While polls show most Americans back an assault weapon ban, influential pro-gun lobby groups such as the National Rifle Association have pressed lawmakers to oppose such a move.
Senator Dianne Feinstein said she might put forward the assault weapons proposal, similar to a previous one she sponsored that expired in 2004, as an amendment to the bill.
But she would need 60 votes from the 100-member Senate to succeed, a margin analysts say the amendment would be unlikely to reach.
“I very much regret it,” Dianne Feinstein said.
“I tried my best.”
The plan had only narrowly passed a Senate panel last week, underlining its poor chances of clearing the full chamber.
It was one of four gun control measures backed by the panel, including expanded background check requirements for people buying guns, harsher punishments for illegal gun trafficking, and more money for security at schools.
Meanwhile, the town of Newtown has seen a surge in applications for gun permits since the massacre at Sandy Hook primary school in which 20 schoolchildren and six teachers were murdered.
There have been 79 requests for gun permits in Newtown since the shooting on December 14, police say, although the town has only issued about 130 licences annually in recent years.
A police official said people were worried about new regulations.
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