It is an acorn nut they are tapered. Take it off throw it away and go to Lowes, Mernards or any place that sells every nut known to man. Take your site post with you and find a nut that fits without having to pay through the nose for it. Reinstall with green loctite.
What does a mag tube do? Not much actually, it just holds shells. CC and FFT both have ti tubes, one is higher priced is it better. I doubt it. I bought CC because of their ObsessiveCompulsiveDisorder over perfection.