Blackheart Crossbow Rail Lube .5 oz.. Reduces center serving wear. Idea viscosity for application. Increased velocity. Scent-free formula. 0.5 oz bottle. Package Contents: Blackheart Crossbow Rail Lube BlackHearts premium rail lube creates a slippery surface for your string to glide across vastly improving shot consistency. Reduces center serving wear. Idea viscosity for application. Increased velocity. Scent-free formula. 0.5 oz bottle.