Expensive but works well. - Aero Precision AR-15 ATLAS Taper Lock Kit

If you have an Aero Atlas handguard you have to buy this little sucker if yours didn't come with one. Most already have this included but some of the kits from Aero that were on sale did not. It works great and really snugs down on the barrel nut. Very well made just ridiculous that it's over $30 bucks. With tax and shipping I think it came to $36.