His job with the Santa Fe ISD Police Department was supposed to be a safe retirement job, allowing him to spend more time at home.
Instead, just four months into his new job at Santa Fe High School, he was faced with the horror of a mass shooter.
He sat down with Eyewitness News explaining the second-by-second details of May 18, 2018.
"When I went down the hallway, I could smell gun powder, and I knew that wasn't right," he said.
Dimitrios Pagourtzis is charged in the deadly shooting that killed 10 people, including eight students and two substitute teachers.
RELATED: Timeline: How the deadly shooting at Santa Fe High School unfolded
Investigators say Pagourtzis brought a shotgun to school and opened fire in the first period art class.
Barnes says he was heading towards the shooter as a fire alarm was sounding.
As he approached the art classes, he says he saw two substitute teachers fall to the ground. Ann Perkins and Flo Rice were each shot.
Perkins was killed, but Rice survived.
"I know he just shot two people and I want to put him down. So, I got up trying to use the corner for cover, and as soon as my arm came around the corner, I got hit," said Barnes. "It was within a second or two after I posted up against that corner. I never saw him, never saw where it came from. I just knew I was hit and backed up."
The shot sent four pellets through his right forearm, ripping through his brachial artery, causing severe blood loss.
"It was coming out of my arm like some kind of horror movie. It was just shooting out of my arm," Barnes recalled.
He credits Officer Gary Forward as the first of many people who saved his life.
"When he was down with me, I kept telling him, 'Just leave me, just leave me,'" said Barnes. "The reason I was saying that was because I could not stand the thought of him getting shot just because I got hit. He put the tourniquet on me and I crawled into the room and waited."
Barnes flatlined twice; first in the Life Flight helicopter and then again in the emergency room.
"I think I lost as much blood as a person can lose and come back from it," Barnes said.
He was released from the hospital on June 20, 2018, 33 days after the shooting.
He has undergone eight surgeries on his arm, and still has at least one more scheduled to remove a metal plate in his elbow.
Since the shooting, Barnes has retraced his steps of survival, meeting and thanking the first responders, pilots, doctors and nurses who saved his life.
"It's horrible, and it's horrific, but at the same time you see people come together and that part of it is uplifting," he said.
Barnes says he was hoping to one day have the option to get back into law enforcement, but his physical limitations with his right hand has likely put an end to his career.