Berger Article on Cartridge Overall Length and Base-to-Ogive
Effects Of Cartridge Over All Length (COAL) And Cartridge Base To Ogive (CBTO) – Part 1
by Bryan Litz for Berger Bullets.
Many shooters are not aware of the dramatic effects that bullet seating depth can have on the pressure and velocity generated by a rifle cartridge. Cartridge Overall Length (COAL) is also a variable that can be used to fine-tune accuracy. It’s also an important consideration for rifles that need to feed rounds through a magazine. In this article, we’ll explore the various effects of COAL, and what choices a shooter can make to maximize the effectiveness of their hand loads.
Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI)
Most loading manuals (including the Berger Manual), present loading data according to SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) standards. SAAMI provides max pressure, COAL and many other specifications for commercial cartridges so that rifle makers, ammo makers, and hand loaders can standardize their products so they all work together. As we’ll see later in this article, these SAAMI standards are in many cases outdated and can dramatically restrict the performance potential of a cartridge.
Bullet seating depth is an important variable in the accuracy equation. In many cases, the SAAMI specified COAL is shorter than what a hand loader wants to load their rounds to for accuracy purposes. In the case where a hand loader seats the bullets longer than SAAMI specified COAL, there are some internal ballistic effects that take place which are important to understand.
Effects of Seating Depth / COAL on Pressure and Velocity
The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long. In fact, the extra powder you can add to a cartridge with the bullet seated long will allow you to achieve greater velocity at the same pressure than a cartridge with a bullet seated short.
When you think about it, it makes good sense. After all, when you seat the bullet out longer and leave more internal case volume for powder, you’re effectively making the cartridge into a bigger cartridge by increasing the size of the combustion chamber. Figure 1 illustrates the extra volume that’s available for powder when the bullet is seated out long.
Before concluding that it’s a good idea to start seating your bullets longer than SAAMI spec length, there are a few things to consider.
Geometry of a Chamber Throat
The chamber in a rifle will have a certain throat length which will dictate how long a bullet can be loaded. The throat is the forward portion of the chamber that has no rifling. The portion of the bullet’s bearing surface that projects out of the case occupies the throat (see Figure 2).
The length of the throat determines how much of the bullet can stick out of the case. When a cartridge is chambered and the bullet encounters the beginning of the rifling, known as the lands, it’s met with hard resistance. This COAL marks the maximum length that a bullet can be seated. When a bullet is seated out to contact the lands, its initial forward motion during ignition is immediately resisted by an engraving force.
Seating a bullet against the lands causes pressures to be elevated noticeably higher than if the bullet were seated just a few thousandths of an inch off the lands.
A very common practice in precision reloading is to establish the COAL for a bullet that’s seated to touch the lands. This is a reference length that the hand loader works from when searching for the optimal seating depth for precision. Many times, the best seating depth is with the bullet touching or very near the lands. However, in some rifles, the best seating depth might be 0.100″ or more off the lands. This is simply a variable the hand loader uses to tune the precision of a rifle.
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Article sourced by EdLongrange. We welcome tips from readers.Similar Posts:
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- Berger Article on COAL and Cartridge Base-to-Ogive PART 2
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Tags: Berger Bullets, Bryan Litz, Cartridge Length, COAL, Jumping bullets, OAL, Seating Depth
These principles explained so well by Mr. Lytz have been included in the QuickLoad program, at least in working principle, even if the numbers are not absolute.
That this program has no capacity to introduce individual jump to the lands numbers, or the pressure increases that seating into the lands at various tensions might generate, may present an insurmountable mathematical challenge for the program, but it is a great starting point.
As Mr Litz so rightly points out, since the original SAAMI standards were developed, so much has changed. Just look at the variances in Shot Start Initiation Pressures (SSIP)for different projectiles today, compared to when most bullets used to be swaged, jacketed lead.
This variance alone has expanded by a factor of six times (6,000 psi!)over the last 20 years, due to changes in materials and designs used in modern projectiles today. I have never seen this issue discussed and would love to see the relationship between SSIP and bullet seating depth explored in greater detail.
Genuinely looking forward to part two and more…
I have been doing this for years except using a slighter slower powder to gain results.
Good article, and one which shows that the COAL is an ineffective term, at best. My own loading terminology is to use the term “OCOAL”, which is the Cartridge Overall Length, as measured to the Ojive. I also use Sinclair Comparitor (nuts), that add about 1 inch to that measurement; the result I document as COCOAL (Comparitor Ojive COAL). It really does not matter where the end of the bullet is, thus COAL is really meaningless, and potentially problematic is comparing the various bullet tapers…
Obviously, COAL does matter for magazine feeding…
RCBS precision mics are a great tool for this. I have a set for every cal. I load.