Burris Smartclip BTC Adapter Black 51mm. Screw on the adapter to your Burris Thermal Clip-On and secure to your optic's objective lens using a quick-detach cam lever. Size is determined by the outside diameter of your optics objective bell. Specifications for Burris Smartclip BTC Adapter: Manufacturer: Burris Color: Black Attachment Type: Clamp Package Contents: Burris Smartclip BTC Adapter Burris 626635: Smartclip BTC Adapter Black 51mm