After charge weight, seating depth is the most powerful accuracy lever you control — and the one most surrounded by jargon. This guide demystifies CBTO, jump, and jam, shows how to find where your bullet meets the rifling, how to run a seating-depth test, and the pressure rules that keep it safe.
In this guide
- COAL vs CBTO
- Jump, jam, and touch
- Finding your touch point
- Running a seating-depth test
- Pressure and safety
- Track CBTO in LoadNode
COAL vs CBTO: why the ogive beats the tip
COAL (cartridge overall length, sometimes OAL) is measured from the case base to the very tip of the bullet. The trouble is bullet tips — especially soft polymer or lead — vary in length, so COAL is a noisy reference. CBTO (cartridge base to ogive) is measured to a point on the bullet’s ogive using a comparator. Since the ogive is the part that actually engages the rifling, CBTO is the consistent, meaningful number for controlling how your bullet meets the lands. Use COAL to check it fits your magazine; use CBTO to tune.
Jump, jam, and touch
When a chambered bullet just contacts the rifling, it is touching the lands. Seat it deeper and the bullet has to travel a short distance — the jump — before it engages the rifling. Seat it out into the rifling and it is jammed. Jump is simply how far off the lands you seat, expressed as a CBTO difference from the touch point. Different bullets prefer different jumps: some shoot best near the lands, others are happy with plenty of jump — it is something you test, not assume.
Finding your touch point
To measure jump you first need your touch CBTO — the length at which the bullet contacts the lands in your chamber. Common methods include a dedicated tool such as a Hornady OAL gauge with a modified case, or a no-tool method using a split-neck or lightly-sized case that lets the bullet be pushed back by the lands. Either way you read the result with a bullet comparator on your calipers and record the CBTO. A reloading resource like Ultimate Reloader walks through the tooling in detail.
Running a seating-depth test
- Settle on a charge weight first (seating depth is tuned after the charge).
- From your touch CBTO, load small batches at increasing jump — for a coarse scan, steps of around 0.010–0.020" cover ground quickly; for fine tuning, 0.003–0.005" steps.
- Shoot a group at each seating depth under consistent conditions.
- Compare group size and shape — look for a depth where the rifle clearly tightens up.
- Confirm the winner with a second test on another day before committing.
Change only seating depth during the test — if you move the charge too, you will not know which helped.
Pressure and safety
Seating closer to or into the lands raises pressure, sometimes sharply. Never pair a maximum charge with a jam into the lands, and back the charge off when testing near the lands, then watch for pressure signs as you work back up. Remember too that magazine length caps how far out you can seat if you feed from the magazine. As always, start from current published data and work up carefully.
Track CBTO and jump in LoadNode
LoadNode stores your measured jam (touch) CBTO, the jump you choose, and computes the resulting loaded CBTO for each load job — so your seating-depth experiments are recorded, not guessed at from memory. It also tracks throat erosion over the barrel’s life, which matters because your touch point moves forward as the throat wears, so last season’s jump is not this season’s. Pair it with measuring each group in MOA to judge the results objectively.
Handloading is an adult activity. LoadNode is a logbook and analysis tool — it never provides load data. Always develop loads from current published data, start low, and work up safely.
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