The 9" Samson Evolution Series Handguard might be able to fit a 16" barrel with front sight post. You will need to remove the front sight post in order to install the handguard, but you should have enough room for the front sight after install. However, the manufacturer does stat the handguard is not compatible with bayonet lugs and sling loops.
The Samson Evolution 10in Series Handguard does not have a QD sling mount.
Samson says this product works with any Milspec barrel nut.
Yes this will work on the Ruger AR-556.
No, this handguard will not work on the Sig 522.
Yes, this rail will work for your application.
Yes, it should work no problem.
It does NOT include a barrel nut (compatible with any MIL-SPEC barrel nut).
Samson Evolution 10in Series Handguard is compatible with any MIL-SPEC barrel nut.
Samson says "Thermal bushings clip to any Mil-spec barrel nut"
Mark H.