Bearing Surface
The length of a bullet's full-diameter shank that actually contacts the bore and engages the rifling as it travels down the barrel.
The bearing surface is the cylindrical portion of a bullet at full caliber diameter, running from where the ogive ends back to the start of the boat-tail or the base. This is the band that rides the bore and into which the rifling cuts to impart spin. Its length governs how much friction the bullet generates and how it obturates and seals against the bore.
Bearing surface length matters for consistency because variation in it from bullet to bullet changes the engraving force and therefore the pressure and velocity. It also interacts with seating depth: a long bearing surface seated deep can intrude on powder space, while seating it out far enough to reach the rifling changes how much of that surface engages at the start of the shot. Match shooters often sort bullets by bearing surface to reduce velocity spread.