If you just want to neck size back your full length die out a turn or two and you have a neck sizing die. However, the chambers on the mosen are usually not round and you may have a hard time getting the round back in, unless you orient each round. Try putting a round back in that you haven't resized.
If you are looking for accuracy you may want to get some cork gaskets and place them in the stock at the pressure points (barrel bands) and under the action at the screws, polish the trigger with an emery board, and you will have a fine shooting gun.
http://www.smith-sights.com/article-...sin-nagant.php