Cartridge overall length

The cartridge overall length (COL), designated L6 in the CIP standard, is a key reloading parameter. The cartridge overall length determines how deeply the projectile is seated in the case. Accordingly, a change in the cartridge overall length always results in a change in the usable case volume and thus a modification of the load. We can illustrate this relationship with a short example calculation for calibre 7 mm-08 Rem in QuickLOAD. The bullet Sierra HPBT 1930 168gr was used and a barrel length of 600mm was assumed.

Cartridge overall length
[mm]
Powder typeLoad
[g] and [gr]
V0
[m/s]
Pressure
[bar]
Remark
71.12RS502.56 / 39.57423308Initial position
68.00RS502.56 / 39.57563597Reduced L6
68.00RS502.48 / 38.47363311Modified load

If the load is not adapted, 289 bar, or +8.7% more pressure, is generated in our example. The load must be reduced by 1.1 grain to bring the pressure back down to the original level. This example shows that a change in the cartridge overall length has a rather significant influence on the internal ballistic parameters.

However, the cartridge overall length also determines the distance between the bullet and the rifling. Shorter cartridge lengths mean that the bullets must travel in free flight before they are pressed into the rifling. A longer free flight allows the gases to flow past the bullet. This can result in increased erosion in the chamber cone. Longer cartridge lengths mean that the bullet is seated less securely in the case. In some cases, the bullet and case mouth are only in contact over a few millimeters; the pull-out resistance is thus correspondingly less. In this case, the bullet’s closer proximity to the rifling is advantages, as the free-flight distance is shorter. There are disadvantages with respect to weapon function: It is much more difficult to correctly transport a longer-than-standard cartridge from the magazine into the chamber. The CIP serves as an aid for weapon and ammunition manufacturers. This norm specifies the cartridge overall length under the parameter L6. You can find the norm via the following link:

http://www.cip-bobp.org/homologation/en/tdcc_public

The cartridge overall length is a highly interesting optimisation parameter for reloaders. The parameters discussed above must be kept in mind:

  • Weapon function (in particular correct chambering)
  • Sufficiently secure seating of the bullet in the case
  • Preventing excessive free flight (increased erosion, possibly reduced precision)
  • The bullet should not be pressed into the rifling when chambered.

As you can see, the requirements are varied, and the cartridge overall length has a major influence in reloading. For instance, the 6mm BR community swears by cartridge overall lengths that put the bullet close to the rifling. Accordingly, the CIP specification of 62.00mm for calibre 6mm BR Norma is just window dressing for these shooters. Everyone exceeds this dimension. There are also plenty of examples in the other direction: 8 x 57 IS (8mm Mauser) is specified with 82.00mm in CIP. However, this length precludes any reliable bullet seating, so we often see cartridge overall lengths in the range 70 to 80mm here. .303 British is another example in this direction. We could probably go on indefinitely on the important issue of cartridge overall length. I certainly wish you a lot of satisfaction in your experiments to find the optimum cartridge overall length. In my opinion, the cartridge overall length is an extremely interesting and important parameter.

Dominik Antenen