5/16" body 100° countersink, 6 flutes.
A countersink cuts the tapered seat that lets a flat-head screw finish flush with the surface. This one has a 5/16" cutting body on a 1/4" shank, 2-1/2" overall, ground to a 100° included angle.
The "chatterless" grind stays in contact with the work rather than skipping across it, which is what stops the tool from cutting a lobed, faceted hole.
100° is the aerospace standard for flush-head fasteners.
