Designed to drill through hard-to-drill materials such as hardened receivers and hardened knife blades. Fractional sizes are usually used for drilling pin, rivet and thong holes in blade tangs while the #28 and #31 are used for drilling receivers for scope mounts. For better results and longer drill life, run at speed specified for each size. Carbide drills are very brittle and must be used with great care. No warranty on this item.