THE 6.5 CREEDMOOR CARTRIDGE MASTER COMPENDIUM » TN-03 — COAL, Jump & Seating Depth Dynamics

How ogive position, magazine length, throat geometry, and bullet design interact to produce predictable precision.

I. Why Seating Depth Matters in 6.5 Creedmoor

6.5 Creedmoor was engineered around long-for-caliber bullets (120–147 gr) with high-BC profiles. These bullets require controlled jump distance to achieve optimal stability and predictable vertical dispersion.

  • Long bearing surfaces
  • Modern secant or hybrid ogives
  • Magazine-fed COAL stability (~2.800″)
  • Low-pressure, consistent seating depth behavior

The result is a cartridge unusually tolerant of factory seating-depth variation.

II. COAL (Cartridge Overall Length) — Functional Boundaries

  • SAAMI max COAL: 2.825″
  • Magazine internal length: typically 2.830–2.850″
  • Throat/freebore: sized for heavy-for-caliber bullets seated with controlled jump
  • Factory match loads: optimized for consistent ogive-to-lands distance across chambers

Creedmoor does not require “touching the lands” to shoot well — its chamber blueprint rewards controlled jump.

III. Jump Distance — What Actually Happens

Ogive jump influences:

  • Initial pressure rise rate
  • Barrel time
  • Yaw behavior
  • Vertical dispersion

Typical optimal ranges:

  • 0.020–0.060″ jump for most high-BC bullets
  • Minimal BTO variation is more important than exact COAL
  • Jump-tuning yields modest ES/SD improvements

This is a core reason factory ELD-M loads earned their reputation for “plug-and-play” precision.

IV. Magazine Constraints & Why Creedmoor Solved the .260 Problem

.260 Remington’s problem:

  • Deep seating required to fit standard magazines
  • Reduced usable case capacity
  • Increased pressure
  • Greater velocity variation

Creedmoor’s solution:

  • Shorter case with a longer neck
  • Optimized throat for 140–147gr bullets
  • Better alignment for high-BC ogives

This is why Creedmoor became the dominant AR-10 precision cartridge.

V. Practical Accuracy Behavior

  • Factory ammunition: typically optimized for 0.030–0.050″ jump
  • Handloaders: often find broad, forgiving nodes (±0.003–0.005″)
  • Gas guns: prefer larger jump for reliability (0.050–0.070″)
  • Bolt guns: may show tight nodes around 0.015–0.035″

Creedmoor’s geometry allows consistent accuracy even with slight ogive variation from bullet to bullet.

Specifications

  • Technical Note: TN-03 — COAL, Jump & Seating Depth Dynamics
  • Compendium: 6.5 Creedmoor Cartridge Master Compendium
  • Focus: Ogive jump, COAL limits, magazine geometry, seating sensitivity
  • Related Topics: TN-01 Case Geometry, TN-02 Pressure Curve, TN-08 External Ballistics