Ranger Michael Maguire was on his lunch break in a "safe area" at the range and had taken off his helmet when he was hit in the head by a bullet, an inquest into his death heard.
He had joined the Royal Irish Regiment in 2010, and survived a tour in Afghanistan.
The inquest in Cardiff heard shooting targets had been set up wrongly and were in "dangerous positions."
It was told the cause of death was a gunshot wound to the head.
The jury returned a verdict of unlawful killing.
Investigations by the military and the Health and Safety Executive are underway.
An inquest recorded its verdict after hearing Ranger Michael Maguire was hit by a stray bullet when targets where set up in the wrong place.
An inquest into the death of Ranger Michael Maguire, 21, who died at an army range in Pembrokeshire, has started today.