I have to agree that NCIS going after this Marine in a federal court is a bit much, his discharge and some kind of punishment should have been plenty.
Had he brought it home and committed a crime with the weapon it would be different but it seems it was a war souvenir. As was mentioned up thread it was a common practice in earlier wars and not always legal. A friend of my Dad, a Marine, brought an M2 Carbine (full auto) back from WWII from the So Pacific and an uncle shipped 8mm Mausers to his four brothers from Germany at the end of the war.
It will not be justice in my view if this Marine is sentenced to a federal prison for this.
While I understand the sentiment about WWII souvenirs, times changed.
More importantly, it is an extremely clear order. There is no member of the military who is not aware of it. It is repeated always and everywhere, driven home to the point of being a pain to listen to it again.
There is absolutely no way the most junior private would be unaware of it. For any kind of long-term, senior NCO, Warrant, or Officer to violate it is the very definition of direct, knowing, disobedience of a standing order.
I have no sympathy in that regard.
OTOH, 10 years is ridiculous.