I'm finally going to finish my Ar. I bough a lower parts kit w/o grip from Anderson. A rifle buffer tube spring and buffer from Areopercison. A fixed Magpul rifle lenth stock. From what I read about the products a castle nut isn't listed anywhere. Is it implied to be in one of them or do I have to buy one? I don't want to get all my parts Tuesday and have to wait another week because I had to buy the castle nut separate.