Charlton is scheduled to be sentenced Dec. 20 in 4th District Court in Nephi.
Gustin said she will request no jail time for Charlton, while Juab County prosecutors will make no recommendation.
"He is punishing himself enough," Gustin said. "He will be punishing himself for the rest of his life for this ... This is the first step toward ending this nightmare for the family."
Charlton sobbed through most of an eight-hour preliminary hearing last month, as a judge heard evidence of the May 28 shooting.
Gustin said Charlton's emotions were a little more subdued during Tuesday's hearing, but he still broke into tears as he pleaded guilty to his brother's death.
At the preliminary hearing, Jonathan Hummell, an eyewitness to the shooting, said Eric Charlton had consumed two glasses of Captain Morgan rum mixed with cola, and the younger boys had also been drinking that night around a campfire.
The group had been telling ghost stories and became spooked when they heard coyotes in the distance. Eric Charlton, a former Marine, grabbed his .45 caliber handgun.
Hummell testified that Charlton told his younger brother, "You know you're my brother when you can trust me with this," while swinging his arm across his body. The gun fired, and a bullet passed through Cameron's left temple.
Charlton had told law enforcement that he initially removed the magazine from the gun, and even pulled the slide back to show the teens that the weapon was empty earlier in the evening, court documents state. He showed the two teens different firearm techniques before replacing the magazine. He told police, however, that he did not remember charging the gun's chamber with a live round.
Gustin said it was possible that if the magazine was slammed into the gun with too much force, it could load a bullet into the chamber without the operator's knowledge.
Justin Bechaver, with the state's crime lab, testified that about 30 percent of the time he forcefully loaded a magazine on Charlton's 1911 Springfield, a bullet was pushed into the chamber.
Prosecutors had originally charged Eric Charlton with a second-degree felony. But Judge James Brady ruled that prosecutors had not met the burden to prove that Charlton acted recklessly when he accidentally fired the shot that killed his brother. He reduced the count to a class A misdemeanor. Prosecutors later filed a motion asking the judge to reconsider his ruling.