Try a round with a heavier bullet and/or less muzzle velocity; 7.62x54r.net is your friend.
Adjusting the rear sight would probably be an idea, too; IIRC, they start out set to around 100 arshins (or metres, in the later models) and if the slider got bumped, it could be set to a longer range.
That guy who mentioned the bayonet has a point, too; the carbines with the spring loaded bayonets were sighted in with them extended, and I believe the 91/30s were meant to have the bayonet fixed at all times. Apparently removing it throws the sights off, but that's just what I've heard.
Could be flinching, as well; try getting a friend to load the magazine with a mix of live rounds and empty cases; that should help you figure out if it's flinch or something with the rifle.