The bayonet for the Model 39 was not a generic type.It was manufactured only for that rifle.The originals are quite expensive,running around $300 but you can find reproductions for around $50 or so.They resemble a large hunting knife.InterOrdinance Corp. was that last company I remember seeing any advertised from.
Incidentally,the rifle is referrred to more commonly as a Mosin rather than a Nagant.When you refer to a firearm as a Nagant,people think you are referring to the Nagant pistol.
The model 39 is generally accepted as one of the more accurate of the Mosins and many think it is on a par for accuracy with the Swiss K31.
