Mike's info is correct, but I have some info to add.
IIRC all Westinghouse Mosin-Nagants are marked "1915" regardless of the actual year of manufacture. They were made from 1915 to 1917.
IIRC the Imperial Russian contracts with Westinghouse and Remington were scheduled to be over well before 1917, but both contracts proceeded far behind schedule due to various problems, notably engineering drawings that contradicted one another, Russian-provided sample rifles that didn't match any of the drawings, and excessively picky and arrogant Russian military inspectors. (According to stories I've read, they were real prima donnas, known for behavior like snatching an unfinished rifle stock from an American production line worker, smashing it, then loudly berating the worker for shoddy workmanship because it broke.
) Both companies had several hundred thousand rifles on hand, unpaid and undelivered, when the October Revolution occurred. The Bolshevik government declared that the rifles weren't needed, partially due to a lack of cash and partially due to an idealistic notion that Communist revolution would soon sweep Europe, ending class warfare and ushering in an era of peace. (How'd that work out?
)
This threatened both companies with bankruptcy, and the US federal government quickly realized that having two of America's largest arms manufacturers go belly-up right at the start of their intervention in WWI would be a Very Bad Thing.
The government wound up buying all of the rifles at a greatly reduced price. Some were issued to US Army soldiers for training, but the soldiers didn't like them and the logistics commanders balked, so most of the rifles wound up being resold to civilians as surplus, usually for very low prices. Many were bought by distributors and subsequently rechambered in .30-06, which, as Mike implies, is an unsafe conversion. Such rifles SHOULD NOT BE FIRED.
AFAIK the only American combat use of the rifles occurred during two generally unsuccessful and pointless WWI-era US military interventions in Russia, particularly the 1918 ANREF expedition to Archangelsk. IIRC the ANREF force was almost entirely equipped with US-built Mosin-Nagants because their main mission was to protect a large stockpile of ammo and other war material in the city- a stockpile they were expected to be able to utilize, but the Bolsheviks removed all of it before US and British forces even arrived.
http://en.wikipedia.org/wiki/Polar_Bear_Expedition
http://en.wikipedia.org/wiki/American_Expeditionary_Force_Siberia
ANREF forces spent a bitterly cold winter in one of the most remote cities in the world, with no real mission, very limited supplies, and a woefully inadequate number of troops to launch any sort of effective attack on the enemy. Their mission quickly became "Maintain a Defensive Perimeter and Try Not to Freeze or Starve to Death".
IIRC they were understandably so demoralized by the time they were ordered to leave that they abandoned almost all of their equipment and weaponry in Russia.
