Bench 6

Seating

Seat the bullet to a safe, repeatable cartridge overall length before tuning seating depth for your rifle.

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Every bench matters. Basics returns the work to the SAAMI baseline. · Control and Tune add measurement depth when the work calls for it.
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Bench 6 Basics
Seating
What you're doing and why
Bench 6 starts with COAL: Cartridge Overall Length. COAL is the total length of the finished cartridge from the case head to the bullet tip. SAAMI defines cartridge and chamber dimensional standards, and published load data gives the basic COAL starting point for safe fit and function. In Basics, the job is to seat the bullet to a verified COAL before rifle-specific CBTO or jump-to-lands tuning begins.
1
Start with COAL

COAL means Cartridge Overall Length. It is measured from the case head to the bullet tip on the finished cartridge. For the basic seating path, start with the COAL listed in trusted load data for your cartridge and bullet. That published length is the SAAMI/factory-standard baseline for safe fit and function, not the final precision-tuning answer.

You'll need Published load data + calipers
2
Set the seating die

Set up the seating die according to the die manufacturer's instructions. Seat a dummy round or first test round, then measure COAL with calipers from the flat case head to the bullet tip. Adjust the seating stem in small steps until the cartridge reaches the target COAL.

You'll need Seating die + calipers
3
Seat with a consistent press stroke

Seat each bullet with the same smooth press stroke. Inconsistent handle pressure, mixed brass, damaged case mouths, or changing neck tension can all change how the bullet seats. If seating feel changes sharply, stop and inspect the case before continuing.

You'll need Prepared charged cases + bullets
4
Verify COAL across the batch

Measure a sample of finished rounds from the case head to the bullet tip and confirm they match the target COAL. Bullet tips can vary slightly, so COAL is not the most precise tuning measurement, but it is still the basic safety, magazine-fit, and function check. Any round that is unusually long, unusually short, or does not match the batch should be pulled and investigated before it moves forward.

You'll need Calipers + finished rounds
Bench 6 Control
Seating
What you're controlling
Bench 6 control moves from COAL to CBTO: Cartridge Base To Ogive. COAL is still the basic safety and magazine-fit check, but it measures to the bullet tip. Bullet tips vary, so COAL is not precise enough to control seating depth for precision ammunition. CBTO uses a bullet comparator to measure from the case head to a repeatable point on the bullet ogive. That lets the shooter define seating depth as a controlled relationship to the rifling.
C1
CBTO
Establish the CBTO @ lands reference

Use the specific bullet you intent to reload for paired with a modified case of the same catrdige/case, bullet comparator, and a Sigma8 CBTO seating gauge and calipers to establish the CBTO measurement where that bullet ogive contacts the lands. This is not a universal cartridge number. It is a specific and individualized rifle chamber/throat ↔ bullet reference point.

This measured seating depth where the bullet ogive touches the rifling is the 0.000 (zero) reference point.

The goal is to measure the exact seating depth where the bullet ogive first touches the lands of the barrel with the cartridge seated inside the chamber.

You'll need CBTO Seating Gauge + Modified Brass + Bullet + Bullet comparator + Calipers
C2
Seating Depth
Set Target Bullet Jump to Lands

Jump is the distance the bullet travels before the ogive contacts the rifling. Once the CBTO at the lands is known, choose a desired jump and subtract it from the lands reference to create the target CBTO for the batch. A 0.020" jump is a common starting point, but it is not universal. Confirm magazine fit and make seating-depth changes deliberately because moving closer to the lands can affect pressure.

Equation:
Target CBTO = CBTO @ Lands - Desired Jump
Example:
3.230 Target CBTO = 3.250 CBTO @ Lands - .020 Jump
You'll need Bullet comparator + calipers
C3
Batch Spread
Track CBTO variation

Measure finished rounds across the batch and record the shortest and longest CBTO values. A small CBTO spread means the seating operation is being held consistently. A large spread can point to die movement, inconsistent press stroke, variations in bullet ogive, neck-tension variation, case-mouth issues, or brass that should have been controlled earlier in the process.

Equation:
CBTO Spread = CBTO Max - CBTO Min
Example:
2.245" - 2.241" = 0.004"
You'll need Comparator + calipers + finished rounds
Bench 6 Tune
Seating
Tune doctrine
Seating-depth tuning changes bullet jump in controlled CBTO steps. Once powder charge and the loading process are stable, CBTO becomes a rifle-response variable: small seating-depth changes can tighten or open groups even when velocity looks similar.
T1
Tune seating depth by CBTO

Do not tune seating depth by chasing COAL. Use CBTO and change one seating-depth variable at a time. Start from a safe, functional CBTO, test deliberate jump changes, and keep powder charge, brass, primer, bullet, and loading process fixed. Qual8 maps CBTO against group size and velocity behavior so the shooter can see whether seating depth is improving the load or just adding noise.

You'll need CBTO records + target data + velocity data
Logbook Record Layer

- Target COAL and actual average measured
- Jump-to-lands measurement (if taken)
- Die setting used (stem position)
- Any rounds flagged for re-seating
- Bench date and session number

Bench walkthrough
Watch the bench in practice
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Bench 6
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Bullet seating walkthrough — die setup, COAL measurement, and jump-to-lands
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