Shanita Love said she was in an SUV with Atwater, Lovette, and another man as they drove around disposing of the weapon.
"Alvin [Lovette] had taken the gun apart and dropped a piece down in the drain," said Love.
Carson was abducted from her home near the UNC campus in March, 2008. Her bullet-riddled body was later found dumped on a Chapel Hill street.
Last year, Atwater pleaded guilty to federal carjacking charges and state charges that included first-degree murder, first-degree kidnapping, and armed robbery. The plea agreements spared him a death sentence but sent him to prison for life.
Lovette faces the same fate if he's convicted. The death penalty is not an option because he was 17 at the time of the killing. He's charged with first-degree murder in the Carson case.
Love said the decision to get rid of the gun came after she saw Atwater tell Lovette his picture was on the news. At the time, police were circulating a surveillance photo from an ATM purportedly showing someone using Carson's ATM card to make a withdrawal.
Prosecutor Jim Woodall showed Love a gun recovered by detectives from the locations she told them they would find parts of the gun. She told jurors that was the gun she saw Lovette and Atwater get rid of.
Woodall said in his opening statement that a sawed-off shotgun was also used in Carson's murder. Love testifed she also witnessed Atwater and Lovette attempting to destroy that weapon before presumably dumping it somewhere.
"Took it outside and tried to break it up on the bricks outside," she said.
An autopsy revealed that Carson - originally from Athens, GA - was shot five times. It was the shotgun blast that ultimately killed her.
Perhaps the most dramatic testimony from Love Friday came when she told jurors how Lovette described Carson's death.
"Demario and Alvin were talking about it and Alvin stated that he had hit her a couple of times and that she was still talking and moving around. And Demario added in that that's when he came," said Love.
She went on to explain that "hit" was slang for shooting.
Lovette's defense has so far pointed all the blame at Atwater and said there is no forensic evident linking Lovette to the actual killing.
On cross examination, Lovette's lawyer Karen Bethea-Shields asked Love about the sequence of her questioning sessions with police. Love testified that she didn't tell police everything she knew at first, but later came to trust one officer.
Bethea-Shields pointed out that Love didn't open up and begin talking about guns until after police told her she could be charged as an accessory to murder and lose her children.
Lovette is also seperately accused in taking part in the killing of Duke University graduate student Abhijit Mahato in January 2008 along with Stephen Lavance Oates. That case has not made it to court.
Mahato was a 29-year-old doctoral student in computational mechanics originally from Tatangar, India. He was fatally shot inside his apartment near Duke's campus. His wallet, cell phone, and iPod was taken.