Sergeant Danny Nightingale, 38, said he had no regrets over fighting the case after being found guilty of two charges of possessing a Glock 9mm and 338 rounds by a court martial board.
He has said he suffered a brain injury during a charity endurance marathon in October 2009 and could not remember having the gun or the ammunition.
Sentencing was adjourned, but Nightingale could be sent to the Military Corrective Training Centre in Colchester, Essex.
He had originally pleaded guilty to the two charges last year and was sentenced to 18 months’ military detention.
But the father of two, from Crewe, in Cheshire, appealed against that sentence and it was reduced to 12 months’ imprisonment, suspended for a year.
He then fought to overturn his convictions and won a fresh court martial, which yesterday found him guilty after four hours of deliberations.
“I never regret fighting it, no. No, not for one second,” Nightingale said outside the retrial in Wiltshire, after being released on bail.
He said the past two years had been “shocking” and described his position as “David versus Goliath”, but added that he had expected to be cleared.
“I believed so, the family believed so, the team believed so. Yes, it came as a great shock,” he said. “If I did not have such a strong family, I’d be broken.”
Nightingale said he would take advice on whether he could appeal, but said his family was facing financial ruin because of the legal costs.
“The big struggle we have got is that it is hugely financial, it is debilitating for us – it is crippling the family,” he said.
“We have to look at whether we can keep going, but I am very lucky. Sally [his wife] has been amazing throughout, as have the rest of the family.”
Nightingale, who is to be medically discharged after an 18-year career, including 12 years in the SAS, also thanked the public saying “99.99 per cent have supported us”.
Mrs Nightingale, who spoke while tightly holding her husband’s hand, said she would be standing by him. “He isn’t guilty and he is not a criminal,” she added. “He never brought a weapon into the country, he never had a weapon in his possession.
“We know the truth, the family know the truth and we know most of the public know the truth, and unfortunately we have not got the result we should have.
“No matter what was said in that court, I live with him and he still struggles with his brain injury on a day-to-day basis.”
The week-long court martial heard that civilian police discovered the haul in September 2011 at the rented house Nightingale shared with another SAS soldier, known only as Soldier N.
Police were acting on a tip-off from Soldier N’s estranged wife who said there might be a gun stored at the house.
The pistol was found in Nightingale’s wardrobe and the ammunition was under his bed in a plastic box.
Nightingale, who was serving in Afghanistan at the time, said that he had no knowledge of them being in his bedroom and said someone else had put them there. “I have no recollection of receiving the gun,” he told the court martial. He said he was a “diligent” soldier who would not have “randomly” stored ammunition at home.
The Crown said Nightingale had put the public at risk by having the lethal arsenal stored in an insecure house.
Prosecutor Timothy Cray told the five-person board: “No soldier, no matter what his experience is or what unit he is attached to, is above the law.”
Judge advocate general Jeff Blackett adjourned sentencing until the Court of Appeal had ruled on whether the court martial board could impose a greater punishment than the one given by the appeal board last year.
Possession of a prohibited firearm carries a five-year minimum term of imprisonment.