Figure 1. Figure 1. This allows the cartridge to produce a higher muzzle velocity at the same pressure. Effects of Cartridge Overall Length (COAL), and Cartridge Base to Ogive (CBTO) Part 1Bryan Litz, Berger Bullets
Many shooters do not realize the dramatic impact that bullet seating depth has on the pressure and velocity generated in a rifle cartridge. Cartridge overall length (COAL) can also be used to fine tune accuracy. This is also a consideration for rifles which need to feed the rounds through a magazine. In this article, we will explore the effects of COAL and what choices a gunner can make to maximize their hand loads. Sporting Arms and Ammunition Manufacturers’ Institute
The Berger Manual, as well as most other loading manuals, present data according to SAAMI standards (Sporting Arms and Ammunition Manufacturers’ Institute). SAAMI provides maximum pressure, COAL, and many other specifications to commercial cartridges. This allows rifle makers, hand loaders, and ammo manufacturers to standardize their products and ensure they work together. These SAAMI standards can be outdated, and as we will see in the article, they can limit a cartridge’s performance. The bullet seating depth is a critical variable in accuracy calculations. In many cases, SAAMI’s COAL is shorter that what a handloader wants to load his rounds to for accuracy. When a handloader seats the bullets deeper than the SAAMI-specified COAL, some internal ballistic effects occur that are important to know. Effects of Seating depth / COAL on Velocity and Pressure
The primary effect of loading the cartridge long is to leave more internal volume in the cartridge. This extra volume inside the cartridge has a well-known effect. For a given powder charge there will be less velocity and pressure produced due to the extra empty space. You can also look at it as you need to use more powder in order to achieve the same velocity and pressure when the bullet is long. The extra powder that you can add to a long-seated bullet cartridge will allow you to achieve a higher velocity with the same pressure as a cartridge where the bullet is seated short. It makes sense when you think about it. When you seat the powder longer and leave more case volume inside the cartridge, you are effectively increasing the size of a cartridge. Figure 1 shows the extra volume available for powder when a bullet is seated long. There are a few factors to consider before deciding that seating bullets longer than SAAMI specs is a good idea. Geometry of a Chamber Throat
The rifle chamber will have a specific throat length that will determine how long a cartridge can be loaded. The throat is that portion of the chamber forward without rifling. The throat is the portion of the bearing surface of the bullet that protrudes out of the case (see Figure 2). The length of a throat determines the amount of bullet that can protrude from the case. When a cartridge’s chamber is opened, the bullet will encounter the lands (the beginning of the rifling). This will cause a lot of resistance. This COAL marks maximum bullet seating length. When a bullet contacts the lands when seated, an engraving force immediately resists its initial forward movement during ignition. The pressures are noticeably higher when a bullet is seated against the lands than if it were seated a few thousandths off the lands. In precision reloading, it is common to determine the COAL of a bullet seated so that the lands are touched. This is the reference length the hand loader uses to find the optimal seating depth. The best seating depth can often be achieved by having the bullet touch or very close to the lands. In some rifles the best seating depth may be 0.100″ or more away from the lands. This is a simple variable that the hand loader uses in order to tune the accuracy of a firearm. CLICK HERE for the Full Article with More Information
