The barrel nut is not included. This uses the factory barrel nut on the Sig M400 Tread rifle. Unfortunately Sig does not sell the barrel nut separately at this time.
I cannot find a description from any vendor that indicates the Tread handguards come with a barrel nut. However, I was surprised that my order from Optics Planet (15" model) which arrived on 12/09/20 had not only a barrel nut but also a barrel nut wrench in the handguard package. I am curious as to how this happened and I doubt that is standard. Nonetheless, I will take it.
IT DOES NOT COME WITH A BARREL NUT AND SIG DOES NOT OFFER A BARREL NUT FOR SALE AT THIS TIME. 01/17/2019 AIM SPORTS OFFERS A REPLACEMENT BARREL NUT (AR556BN) AND SCREWS (ARHGMS) BUT I DONT KNOW IF IT WILL WORK
Mine (13") that I received today, 1/17/24, DID come with both the barrel nut and required wrench.