Rifleman Tom Lindsay, now 17, and Stuart Weir, 16, paused during target practice on a rifle range at Ponteland, Northumberland, last September, when they pointed their weapons at each other.
Rifleman Lindsay squeezed the trigger unaware his automatic rifle had a bullet in the chamber - and Stuart fell dead from a head wound.
Rifleman Lindsay's mother Christina, from Inverness, said: 'It's a great relief. It was a tragic accident. They were the best of mates and nobody was more shocked and upset than Tom.'
During the four-day trial her son told how he is still haunted by the shooting of Stuart, from Lanarkshire. Together they had joined the junior infantry battalion of the Scottish and King's Division two months earlier.
An Army spokesman said Rifleman Lindsay had continued his training at Albemarle Barracks, near Newcastle, while on bail and his military career would go on.