The bourrelet is the forward part of the sabot, supporting the front of the projectile
The bourrelet is identified by the white line around the circumference of the projectile
The bourrelet of a projectile is the part or section of the projectile having the same diameter as the bore of the barrel through which the projectile is to be propelled. In the case of a sub-caliber projectile, such as an armor piercing round, a sabot surrounds the sub-caliber portion, with driving bands at the rear and the bourrelet supporting the front, aligning the armor-piercing portion with the center of the barrel of the gun. The bourrelet diameter is important in that if it is not a tight fit, the in-bore yaw angle will affect the stability of a projectile during flight.
The original article can be found at Bourrelet and the edit history here.