Gusset for your AR-15 Pistol Build or Hammerhead-equipped rifle. This allows you to use a receiver extension tube and contain the rear takedown detent spring without need for a receiver end plate. Clamps down on the tube and engages with the anti-rotation hole in the rear of the receiver to prevent the buffer tube from unscrewing during use. Works only with 1.1"OD receiver extension tubes (both entry length and A1 length). Will not work with the other pistol buffer tubes we sell. This is the same gusset that's included with the ACE ARUL and ARUL-E stocks.