It is not uncommon for the military to procure a limited number of guns for target shooting, evaluation, or some other special purpose, and they may be marked as "U.S.", or have some similar service branch marking. But there are also many fake markings, put on to enhance the value of the gun.
Anyone planning to buy a non-standard gun with military markings at a price substantially higher than a similar gun without the markings should be very careful. As always, "Buy the gun, not the story."
Jim