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December 10th, 2019

How Changes in Cartridge OAL Can Alter Pressure and Velocity

Figure 1. When the bullet is seated farther out of the case, there is more volume available for powder. This enables the cartridge to generate higher muzzle velocity with the same pressure.

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 As we’ll see later in this article, these SAAMI standards are in many cases outdated and can dramatically restrict the performance potential of a cartridge.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.

Permalink Bullets, Brass, Ammo, Reloading 2 Comments »

November 1st, 2019

Ogives, Meplats, Boat-Tails and Other Bullet Design Elements

Noted gun writer Glen Zediker (author of Top Grade Ammo), regularly contributes tech articles to the Midsouth Shooters Blog. One of Glen’s Midsouth Blog articles covers Bullet Design. We suggest you read the article — even seasoned hand-loaders will learn a few things about projectile properties (and how to choose the right bullet design for your needs). Glen also wrote a recent Blog article on cartridge pressure signs, linked below

Read Zediker Bullet Design Article | Read Zediker Pressure Signs Article

Glen explains: “A ‘match’ bullet’s job is to perforate a piece of paper. A bullet designed for varmint hunting, on the other hand, is designed to produce explosive impact, and one for larger game hunting strives to strike a balance between expansion and penetration. However! No matter how it’s built inside, there are universal elements of any bullet design, and those are found on the outside.”

In his article, Glen identifies the key elements of a bullet and explains how they are defined: “Base, that’s the bottom; boat-tail, or not (flat-base); shank, portion of full-caliber diameter; ogive, the sloping ‘nosecone'; tip, either open or closed (open it’s called the ‘meplat’). The shape of the ogive and the first point of ‘major diameter’ are extremely influential elements. The first point of major diameter can vary from barrel brand to barrel brand because it’s the point on the bullet that coincides with land diameter in the barrel — the first point that will actually contact the barrel as the bullet moves forward. When there’s a cartridge sitting in the rifle chamber, the distance or gap between the first point of major diameter and the lands is called ‘jump’, and, usually, the less there is the better.”

Ogives Analyzed — Tangent vs. Secant Bullet Designs
Glen notes that bullet designs reflect secant or tangent profiles, or a combination of both: “The two essential profiles a bullet can take are ‘secant’ and ‘tangent’. This refers to the shape of the ogive. A tangent is a more rounded, gradual flow toward the tip, while a secant is a more radical step-in, more like a spike. Secants fly with less resistance (less aerodynamic drag), but tangents are [often] more tolerant of jump [or to put it another way, less sensitive to seating depth variations].”

Glen adds: “Ogives are measured in ‘calibers’. That’s pretty simple: an 8-caliber ogive describes an arc that’s 8 times caliber diameter; a 12-caliber is based on a circle that’s 12 times the caliber. The 8 will be a smaller circle than the 12, so, an 8-caliber ogive is more ‘blunt’ or rounded. Bullets with lower-caliber ogives are more tolerant of jump and (usually) shoot better, easier. Higher-caliber ogives [generally] fly better, farther. This is an important component in the ‘high-BC’ designs.”

Learn More in Zediker Books
Glen has authored a number of excellent books for hand-loaders and competitive shooters. Here are three of his most popular titles, including his latest book, Top Grade Ammo: