Well....historically U.S. soldiers love war trophies, as my pops brought back a coupel .303 Enfields from Vietnam. With that said it would be about impossible for a U.S. service person to smuggle back a weapon or other said contraband item.
My guess is that they were sold/given to insurgents.
A third option could be that it is a part of Kosherboys theory regarding another 9/11 plot.