Looking at the timeline the Glock was sent to Hereford (Credenhill actually) in/shortly after November 2007, and then to the house he was sharing (no date given).
His memory loss occurred in 2009 during the trek in Brazil.
There is therefore all of 2008 and a part of 2009 (before the trek) when the Glock was in his care and he knew all about it. Someone sharp enough to hold his rank in the Regiment really ought to have taken more care. He could at least have asked the RSM or similar what to do with it even if he didn't know the rules (which I find surprising).
"But the judge did not believe that he had no recollection of being in possession of the weapon" - which makes perfect sense, in my opinion.