A Cork-born soldier in the British army was killed during a live fire exercise in Wales last year.
Michael Maguire, 21, was hit in the temple by a single machine gun bullet probably fired from 1km away.
The inquest into his death heard that Ranger Maguire, of the 1st Battalion, The Royal Irish Regiment was standing in a designated safe haven beyond where the live fire training was taking place.
Evidence suggests a fellow soldier firing a machine gun at a designated target placed on the range hit Ranger Maguire.
The inquest jury also heard evidence that civilians who may have been relaxing at Freshwater West Beach, near Tenby, could also have been hit.
Captain Gary Palmer, an army weapons expert, investigated the tragedy in parallel with the police to ensure later recruits trained safely.
He said soldiers under the command of Lieutenant Jonathan Price had shot outside the safe areas specified.
In particular, they shot outside a designated area designed to ensure their bullets were no danger to other soldiers on shooting exercises or themselves.
Steps taken to guard against this are called "deconfliction" and appear to have gone seriously wrong during the day.
It meant a machine gunner attacking a static target placed at the range for the exercise shot over land.
Ranger Maguire, who was 1km away without his protective helmet and armour, fell within its range.
The inquest heard that frantic efforts were made to keep him alive following the shooting and that army paramedics were on the scene within two minutes.
He was airlifted to hospital in Cardiff but was pronounced dead within 30 minutes of his arrival.
The inquest is scheduled to run for up to three weeks.