Allan Weir, 25, said he lost his job and needs surgery to save his eye after Calvin Blyth shot a loaded paintball gun at his face.
Blyth was working as a senior marshal at the Ape paintballing site in Ratho when the incident happened.
The 20-year-old claims he thought the gun was not loaded, but failed to check the weapon for any paintballs before he fired it.
And yesterday at Edinburgh Sheriff Court, he was found guilty of culpable and reckless conduct.
Mr Weir, a father-of-one from Gilmerton, had begun working at the site in September last year and was only on his second day as a marshal when the incident happened.
The former Royal Scots infantryman was wearing no protective clothing and the paint pellet smashed into his right eye, crushing his eyeball.
He was left blind in his right eye and needed eight stitches. Now he needs surgery in the hope he can avoid needing a prosthetic eye. He said: "I can't work, I can't drive or use a computer now – it has completely ruined my life.
"I can't do anything I used to enjoy doing."
On September 7 last year, Mr Weir was helping Blyth and another workmate clean and prepare weapons for customers who were coming that afternoon.
Mr Weir checked a weapon was not loaded before discharging the empty chamber at Blyth, blowing the compressed CO2 which powers the guns on his face.
But Blyth, who had been working at the site for 14 months, reacted by picking up a nearby gun and firing it. It turned out to be loaded and the paintball struck Mr Weir on his right eye, knocking him over. He said: "It felt like being hit by a hammer, I thought my eye had been burst."
In court, Blyth claimed he did not realise the gun was loaded, insisting he had picked up one which should already have been checked.
Gordon Reid, site manager of Ape, said it was a one-off. He said: "Health and safety at Ape is paramount.
This sort of incident has never happened before or since and I'm sure Calvin has learned a lesson he will never forget."
Sheriff Michael O'Grady described the incident as a tragic accident but said he had to find Blyth guilty. He said: "
It must have been absolutely clear to everyone that these weapons were capable of causing harm."
He called for reports and Blyth, of Addiston Lodge, Ratho, will return to court next month.