The 17-year-old is one of four recruits to be found dead at the British Army barracks in Surrey.
While three pathologists said the evidence suggested that Pte Collinson's death on 23 March 2002 was suicide, they have all conceded that someone else could have been holding the firearm.
Yesterday the coroner's court heard others say that Pte Collinson had been talking of killing himself that night, but that despite this he had illegally been given a powerful SA80 rifle.
Yvonne and Jim Collinson said it was their first opportunity to hear at first hand from those who were there on the night their son was found with a single bullet wound to the head.
Accompanied by the parents of Geoff Gray, who was found in similar circumstances six months earlier, they renewed their calls for a public inquiry into all the deaths.
"We just hope to find the truth," Mrs Collinson said.
The young Scotsman, who had no history of mental illness, had been in good spirits and looking forward to Easter when his mother dropped him off at the barracks that Saturday afternoon, the inquest jury was told.
Seven hours later he was on guard duty when he went to make a perimeter check. A single shot was heard, and minutes later his body was found with a mortal wound to the head and the rifle beside it.
Yesterday the court heard from three pathologists who said they could find no signs of a struggle and no indication that he had been held down. Under examination from the family's barrister, John Cooper, Dr Susan Dodd conceded, however, that it was only within the past six weeks that she had made a note of no external bruising in her report. Furthermore, despite her reservations, she agreed that someone else could have been holding the rifle.
Later Dr David Rouse said the position of the rifle, the nature of the wound and the lack of bruising led him to believe that Pte Collinson's injuries were self-inflicted. But he agreed with the findings of another pathologist, Dr John Clarke, who said that he could not "entirely rule out someone else held the gun under his chin and fired it without otherwise causing physical harm".
Paramedics who had attended the scene that night also gave evidence, admitting they had assumed that the youngster had killed himself. The ambulance technician Graham Furlonger said in a statement that he had spoken to John Donnolly and Stacey McGrath, both privates, who had been on guard duty with Pte Collinson that night. "I recall one of the soldiers saying something about the soldier who had died had spoken about harming or killing himself that evening when they were on the minibus. He was saying that he heard the soldier say, 'I am going to do it tonight,' or something like that. While I was with the body I heard that another soldier had given the deceased his gun and he had gone on the pretence of having a cigarette and then shot himself."
His colleague Roy Gaskin added that a corporal told him that night that whoever had signed out the weapon and handed it illegally to the youngster would be facing a court martial.
The Surrey coroner, Michael Burgess, reminded the jury that this was an inquest, not a trial, and that they should put everything they had heard about the Army and this barracks out of their minds. He said: "This inquest is entirely and exclusively concerned with [Pte Collinson's] death."
The hearing is expected to last two weeks.