Tap, high carbon steel, 5/16-32 thread, plug tap.
Tap Drill: for 5/16-32 thread, use Letter Drill-I
We recommend high carbon steel taps for gunsmithing. Avoid using common HSS (high speed steel) taps from the hardware store, especially when tapping blind holes.
High Speed Steel (HSS) is a special alloy made to retain an edge at "high speed" (i.e. when hot) as a lathe tool bit. Gunsmith taps are turned manually, never run so fast that they get hot. HSS taps are often more costly.
High carbon steel is sharper than HSS High Speed Steel, which contains nickel.
If broken, Track's high carbon steel gunsmithing taps can be fractured and picked out using a prick punch, often with no damaged to the hole or the part.
If broken, HSS taps must be "burned" out, using an EDM (Electro-Discharge-Machine) tap burner, a costly process!
Be sure to order the correct diameter tap drill and clearance drill bits.