Durable, flexible polyethlene with a textured outer surface. I.D. Specs relate to the smallest diameter inside the bottom of the cap. The FBC11K makes a great cover for the G1 AFTCO gimbal, the FBC12K fits the G2/GP2 gimbals and the ABC8 fits the G3/GP3 gimbals. Length - .65" I.D. - .63" O.D. - .83"