I have a pair of those handguards. Both came with a proprietary barrel nut and proprietary tool. I agree it's a little confusing without instructions. Your barrel nut is that very small castle nut looking piece that threads into the handguard mount. I agree it looks funky, but it works like a charm. I highly recommend you use some antiseize on the thread interface between the nut and that inner thread portion of the mount. Make sure when tightening that you keep a solid hookup between the provided barrel wrench and the castle nut looking teeth on that nut. It's one of the craziest looking barrel nut, handguard mounts I've ever envisioned, but it works like a charm. I wish I'd bought 2 or 3 more of these when CDNN had them on sale for $99. And you're right...there's almost nothing on the internet to describe how this works.