Inner and outer surface, strings made of FiberEasy installation via included lace up shock cord, cord can be fit in the slots of rail to Provent slippery, allow to use rail for accessories.4-layer structure design is for better and faster heat dissipation when blazing away, will not impact on optics using.Friendly for using all accessories, light, laser, scope, grip.Inner and outer rated to 1800 F, heat fast. Protect from burns.