What COAL Actually Measures
COAL is the distance from the base of the cartridge to the tip of the bullet. It sounds useful, but it has a fundamental problem: bullet tips are inconsistent. Even within a single box of match-grade bullets, tip length can vary by several thousandths of an inch — enough to matter at the seating depths precision shooters work with.
If you seat two bullets to the same COAL but their tips differ by .004", their ogives are .004" apart. Their relationship to the lands is .004" different. At jump-to-lands distances precision shooters care about — often .020" or less — this is a meaningful variation that COAL cannot detect.
What CBTO Measures — and Why It's Better
CBTO measures from the base of the cartridge to a specific point on the bullet's ogive — the curved portion of the bullet body, not the tip. The ogive is where the bullet contacts the lands, and it is far more consistent between bullets of the same lot than the tip.
By measuring to a consistent point on the ogive, CBTO gives you a measurement that is:
- Repeatable within a lot of bullets
- Directly related to bullet-to-lands distance
- Sensitive to throat erosion as the barrel ages
- Meaningful for comparing seating depths across load development sessions
CBTO doesn't tell you the absolute distance from bullet to lands — that depends on your specific chamber. But it gives you a consistent, repeatable measurement that you can track over time and compare between sessions.
Throat Erosion and CBTO — The Connection
Here is why CBTO becomes essential for tracking barrel life: as your throat erodes, the lands move. The point where your bullet contacts the rifling gets progressively further away from the chamber. A bullet seated to the same CBTO measurement as six months ago now sits further from the lands than it did then — because the lands moved, not the bullet.
This is why Borely tracks CBTO readings over time. If you establish a baseline measurement when the barrel is new (or at a known round count) and record subsequent measurements at regular intervals, the increasing CBTO number tells you exactly how far your throat has eroded — in thousandths of an inch, as a real measurement rather than an estimate.
The Lands tab in your barrel detail view plots your CBTO readings against round count, showing the drift from your baseline. This gives you both a current erosion picture and a projection of where the throat is headed.
A Critical Warning About CBTO Measurements
CBTO is measured with a comparator — an insert that fits between the calipers and contacts the bullet ogive. And here is where most shooters introduce a critical error: comparator readings are relative, not absolute.
Different comparator inserts of the same nominal size contact the ogive at slightly different points. Different calipers read slightly differently. If you measure your CBTO with a Hornady insert on a Mitutoyo caliper today, and then next session use a Sinclair insert on a different caliper, your numbers are not comparable. You will think your throat has moved when what has actually changed is your measurement system.
To track throat erosion meaningfully with CBTO:
- Use the same comparator insert every time
- Use the same caliper every time
- Record exactly which tools you used with every measurement
- Never change tools mid-barrel without establishing a new baseline
Borely requires you to record comparator and caliper with every CBTO entry precisely because of this — and will warn you if your recorded tools differ from a previous reading.