As odd as it may seem this gun was neither trophy nor memento, but was 'awarded' to this soldier as a mark of respect and indeed honour by a Police Commander in Iraq.
Yes, a gun is a gun and the law is the law, except that
the law is not quite the law.
This is the second time recently where military discipline has crossed-over to the civilian realm with dire consequences.
In both cases the accused was
advised to plead guilty, this along with the mind-set of soldiery - take it on the chin, get it all over with quickly and get back to doing your job etc. works against a soldier much more than against a civilian in broadly similar circumstances.
This may be the original story, which is not part of the link I gave in #1...
http://www.telegraph.co.uk/news/9669410/SAS-war-hero-jailed-after-betrayal.html