Let's put this one to rest: I KNOW they use M14 rifles because 1) I have been there and 2) here is a quote from the link listed above: "As the active sentry nears the conclusion of his walk, a uniformed relief commander enters the plaza to announce the Changing of the Guard. When the sentinel assigned the next walk leaves the guards' quarters, he unlocks the bolt of his M-14 rifle to signify that he is ready to begin the ceremony. The relief commander approaches the tomb, slowly salutes, then faces the visitors and requests silence during the ceremony."