Former SAS sniper Sergeant Danny Nightingale will begin giving evidence in his court martial in Wiltshire later today.
Sergeant Nightingale is accused of the illegal possession of a Glock 9mm pistol and 338 rounds of ammunition, which were given to him after leaving Iraq in 2007.
Earlier, the court martial heard how the firearm and ammunition were in found in the former soldier's accommodation in 2011.
Nightingale's previous conviction for the same charges was quashed by the Court of Appeal on 13 March.
Summarising, on the opening day of Sgt Nightingale's court martial, the prosecution said it was for the board to determine fact and on that basis whether he was guilty or not guilty.
– Timothy Gray, prosecuting at Sgt Danny Nightingale's court martial
We suggest that the questions that the evidence in this case raises and the matter are straightforward.
Did the defendant put the gun and the ammunition in his bedroom or did somebody else put it there?
If the gun and ammunition belonged to somebody else, how did the defendant miss it?
Is the claim of memory loss in respect of the gun and ammunition the truth or a lie told to try and avoid the consequences of the truth?
Sgt Nightingale, has pleaded not guilty to a charge of possession of a prohibited firearm and 338 rounds of ammunition.
Following a search of SAS sniper Sgt Danny Nightingale's accommodation and the discovery of the arsenal, he and Soldier N were both summoned to see a superior officer, a court heard.
Timothy Cray prosecuting said: "Soldier N then told the defendant that he had a pistol in his room and ammunition at the property.
"The defendant then said words to the effect 'I've got exactly the same at the house'."
Soldier N later pleaded guilty to Firearms Act offences after a weapon, ammunition and hand grenade were recovered from the property.
He was given a sentence of military detention, which he is serving.
A court martial has heard that a firearm and ammunition were in found Sgt Danny Nightingale's accommodation, which he shared with a soldier - known as Soldier N - in 2011.
He admitted possession of a firearm and ammunition when he was arrested and interviewed.
"When asked to account for the presence of the items found at his address, he said he had brought the pistol back to the UK in personal baggage as a war trophy," the prosecution said.
In a 2011 interview with the Royal Military Police, Sgt Nightingale "clarified" that he was given the pistol as a personal gift by a group of Iraqi nationals he had been working with and that he intended to give the weapon to his unit as a leaving gift.
"During the interviews he sought to excuse his actions by saying that his failure to comply with the proper procedures was down to oversight", Timothy Cray, prosecuting said.
"If the defendant is claiming that, in 2011, he missed seeing the weapon and the ammunition through oversight or through being away from his base, the Crown suggests that this claim does not stand up to analysis.
Timothy Cray, prosecuting at the Danny Nightingale retrial, has said: "No soldier, no matter what his experience or the unit he is attached to is above the law."
The prosecution say the defence will claim that Sergeant Nightingale's earlier admissions to the offences are unreliable and that someone else could have put the gun in his wardrobe and ammunition under his bed.
Their claim of unreliability is put down to memory difficulties he says he suffered.
A panel of five officers who will sit as board or jury for Danny Nightingale's court martial have been sworn in.
The Judge Advocate General advises them to ignore any media coverage they have seen to date in this case.
Some special forces members currently serving in Afghanistan are expected to give evidence via videolink during this court martial.