Cartridge Base to Ogive
Also: CBTO
Distance from the cartridge base to the curve of the bullet's ogive, measured with a comparator that ignores tip variation. More repeatable than OAL.
The ogive is the curve of the bullet’s nose. A bullet comparator clamps on the ogive at a fixed point, ignoring the tip, so two bullets of the same lot register the same CBTO even if their tips differ by a few thousandths.
Tracking jump (the distance between bullet ogive and the rifling lands) is done in CBTO, not OAL. Once you find a CBTO that groups, you can reload to that number every time.