I own an Adams Arms AR15 that is excellent quality. I am happy with what they make.
Looking at the pictures of the Alpha S, it looks they they are using a clamp-on handguard. It is attached to the barrel nut with friction achieved by tightening those two screws on the bottom. The handguard on my PA10 is attached in the same way. The handguard wants to walk forward under recoil sometimes. Take your tools with you for a while until you get the screws tightened to the point the handguard isn't going to move.
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