SPRINGFIELD — A Georgia man serving a life sentence for the shotgun slayings of his father and brother deserves a new trial because investigators ignored evidence of a possible murder weapon, a gun owned by one of the victims, the convicted killer's attorney told a judge Monday.

Defense attorneys say the 12-gauge shotgun that a family friend turned over to the Effingham County sheriff during the murder trial a year ago could point to a different killer and clear Craig Heidt, who prosecutors say gunned down his own family because they discovered he was sleeping with his brother's wife.

"We think it changes things dramatically because this evidence should have been disclosed to us back when this weapon was turned over," Heidt's attorney, W. Dow Bonds, said after a court hearing Monday.

A jury last December convicted Heidt, 43, of killing his father, real estate developer Philip Heidt, and younger brother, Carey Heidt, as the victims slept in their beds. The defendant's mother survived a shotgun blast to the face, but testified at his trial that she didn't know who shot her.

Prosecutors insist the shotgun being discussed in court Monday has no connection to the slayings. They say authorities knew of the gun for more than a year before the trial and had dismissed it as a potential murder weapon.

"Had this gun ever been talked about at the trial, there's no reasonable probability that the outcome would have been any different," said prosecutor Michael Muldrew.

Superior Court Judge F. Gates Peed did not rule on Heidt's motion for a new trial Monday and gave attorneys until Feb. 15 to file additional written responses.

During Craig Heidt's trial in 2010, prosecutors didn't have a shotgun to present to the jury as evidence. All three victims were shot with 12-gauge buckshot, but the killer didn't leave any empty shell casings.

Heidt, an avid hunter, had 3-inch magnum shells like those used in the slayings inside his truck when investigators searched it. They found he also owned a Remington Model 870 shotgun — a gun that would fire those rounds — but the weapon was missing and never recovered.

In court Monday, Sheriff Jimmy McDuffie testified that a friend of the Heidt family brought a shotgun to him in the final days of the trial last December. He said the serial number identified it as a gun registered to Heidt's slain brother. McDuffie said his records showed that an uncle of the Heidt brothers turned over that same shotgun and four more of the victim's firearms to the sheriff's office for safekeeping in 2009 because the slain man's widow didn't want them in her house. He said all were later returned to her.

The sheriff said investigators performed no forensic tests on the shotgun because they never considered it a potential murder weapon. Still, the sheriff said he went to the courthouse to tell prosecutors during the trial last year that someone had brought the gun back to his office. He said the judge called attorneys from both sides — including Bonds — into a meeting in which they all agreed it didn't warrant delaying the trial.

"Everything was done above board and there was nothing hidden," McDuffie said.

Bonds said he didn't recall any meeting with the judge about the shotgun. He said records of the sheriff's office keeping the gun 2009 should have been included in the case file, but never were.

Walter David Dumas testified he took the gun to the sheriff after he noticed it was the same Remington model prosecutors suspected to be the murder weapon. He said Robin Heidt — the wife of victim Carey Heidt and then-lover of his accused brother — brought him the gun in pieces and asked if he would reassemble it.

Dumas said he was told the shotgun had been taken apart by the woman's brother, John Henry Rast, who committed suicide by hanging himself in September 2010. His death prompted a delay in Heidt's trial.

Bonds said that if Rast can be linked to the shotgun, he might be the real killer.

"That's a theory we can't completely exclude," Bonds said.

Muldrew argued there's no way to identify any specific shotgun as the murder weapon without shell casings. That's because shotguns fire small pellets that don't yield the same telltale markings that allow ballistics experts to match fired bullets to rifles and handguns.

He said also noted it's not surprising that both Heidt and his brother would have owned Remington Model 870 shotguns. On its website, the gun maker calls that model the best-selling shotgun in the world with more than 10 million having been produced.