AR-15 LOWER RECEIVER BUFFER TUBE VISE FIXTURE Keep your buffer tube straight while tightening your castle nut. Here at Brownells, we know how hard it is when tightening the lower receiver castle nut to keep the buffer tube from rotating during building or maintenance of an AR-15 platform. That is why we have developed the AR-15 Lower Receiver Extension Vise Fixture. Made from anodized aluminum, place the fixture in your favorite vice, like the Brownells Bench Vise, and tighten it down. Insert the lower onto the fixture with the buffer tube through the sliding tube support. Move the sliding tube support toward the receiver leaving enough room for an armorer's wrench. Adjust Mfg: Brownells