Cartridge OAL — How It Affects Pressure, Velocity, and Accuracy
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 As we’ll see later in this article, these SAAMI standards are in many cases outdated and can dramatically restrict the performance potential of a cartridge.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:
- How Changing Cartridge OAL Can Alter Pressure and Velocity
- How Cartridge Overall Length (COAL) Affects Pressure & Velocity
- How Changes in Cartridge OAL Can Alter Pressure and Velocity
- Berger Article on Cartridge Overall Length and Base-to-Ogive
- Cartridge Base-to-Ogive (CBTO) Length — Key Considerations
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Does length of bullet effect rifle accuracy? With the same bullet weight does the length of the bullet, long over short, effect the long range accuracy of a 22-250 rifle?
Seems to me that COAL is a bad proxy for a way to measure what is a different issue. The real issue is bullet clearance relative to the leade, or start of the engraving. But the ogive of the bullet can vary from blunt through tangent and secant, each with – for a given COAL – a significantly different distance off the lands.
The jump before the bullet contacts the leade – the tapered portion of the rifling where the engraving starts – makes a difference in the rate of pressure rise when the cartridge is fired. If you use a given COAL with a secant ogive bullet, and switch to a round-nose of the same weight – and use the same COAL – you’re in trouble with chamber pressure. You’ll most likely seat the bullet deeper in the case upon chambering, and cause a pressure excursion because there will be both no jump after firing, and a lower effective case capacity.
Also, I see no reference to the accuracy issue in what is written above. Usually, that is considered in light of a minimum jump between the case and the Leade, so that the bullet longitudinal axis remains coaxial with the barrel longitudinal axis, but with sufficient space to allow movement to begin to avoid overpressure from seating the bullet in contact with the Leade.
Finally, on definitions, SAAMI used to define Leade as: That section of the bore of a rifled gun barrel located immediately ahead of the chamber in which the rifling is conically removed to provide clearance for the seated bullet. Also called Throat or Ball Seat. That is, the tapered portion of the rifling at the beginning of the rifling.
Now the listed definition for Leade has been removed, and the issue is defined at Throat. Throat: The tapered portion of the bore of a barrel, immediately ahead of the chamber which is sized to provide clearance for the bullet of the loaded cartridge. Also referred to as Leade or Ball Seat and is associated with Free Bore.
That conflicts with your diagram above. It also defines Free Bore: A portion of the chamber, usually cylindrical, forward of the casemouth, of a diameter larger than the projectile, in which rifling is not present. See Bullet Jump. See Throat.
And finally, Bullet Jump: The distance that a bullet must travel from its position at rest in the cartridge case to its initial engagement of the rifling. Also Known As: Free Travel
The SAAMI definitions are somewhat of a mess, confessing that there is a jumbled understanding of what the terms mean. For decades, chamber meant the negative space into which the cartridge case fit, with free bore constituting the throat, and the Leade being the tapered portion of the rifling at the end of the throat that began the engraving of the bullet upon firing. The jump was the distance that the bullet traveled after firing before it first contacted the tapered lands of the Leade. Your diagram, which shows the bullet nose in contact with the lands, conflicts with both that understanding and the current SAAMI definitions. See: https://saami.org/saami-glossary/
When ever someone brings up COL or COAL,
there are always people explaining the distance
between bullet and the beginning of the rifling.
Bullet Jump etc.
I and some others are more interested in how
COL affects chamber pressure. This can be
critical with some handguns where .1 grain
of powder can be the difference in a good load
or pressure too high. When loading to Max
one must always be careful not to load the bullet too
far into the case.
Using a 48gr charge of Alliant Reloader 17 behind a 180gr Speer Hotcor SP in the Schmidt-rubin 1911 rifle results in a little over 2600fps when loaded to an OAL of ~3.01″, but just UNDER 2600fps when loaded to an OAL of ~2.95″. I’m having trouble understanding why the shorter OAL is producing a lower velocity using the same powder charge and bullet, from the same rifle. I assume the chamber pressure must have risen, but I’m not seeing the evidence.