5/16 X 7/16 High Speed Steel 2 Flute Solid Cap Screw Counterbore hole

5/16 X 7/16″ Shank High Speed Steel 2 Flute Solid Cap Screw Counterbore
For socket head cap screws.
Straight shank.
Pilot size is 5/16″.
Cutting diameter is 15/32″.
Overall length is 4″.

A counterbore is a cylindrical flat-bottomed hole that enlarges another coaxial hole, or the tool used to create that feature. A counterbore hole is typically used when a fastener, such as a socket head cap screw or fillister head screw, is required to sit flush with or below the level of a workpiece’s surface. Source Wikipedia

What is the difference between countersink and counterbore?

The difference between them is the angle at which the larger hole is drilled. A countersink’s larger hole is tapered at an angle; whereas a counterbore is drilled straight into the material and leaves a flat bottom between the counterbore and the narrower inner shaft.