SAAMI was founded in 1926 at the behest of the US government, with a charter to create standards, coordinate technical data, and promote firearms safety.
The Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI, pronounced ”Sammy”) is an association of American firearms and ammunition manufacturers. SAAMI publishes various industry standards related to the field, including fire code, ammunition and chamber specifications, and acceptable chamber pressure.
Only manufacturers that are members of SAAMI are bound by the Institute’s guidelines.
SAAMI’s work is broken up in various committees each with a specific charter.
The Technical Committee does the main work of SAAMI. It is their job to set standards for ammunition and firearms. They interface with their European counterpart C.I.P. to try to develop common, internationally recognized standards. The technical committee provides an industry glossary to facilitate better communication.
The Logistics and Regulatory Affairs committee (also called SLARAC) is responsible for helping create transportation and store regulations. This is done mostly through educating people and agencies on safe practices.
They work with and are members of Dangerous Good Advisory Council, International Society of Explosive Engineers etc.
The Environmental Committee works on science-based management of environmental issues such as wildlife, conservation, and human health as they relate to products produced by SAAMI member companies.
Their goal is ”A clean and healthy ecosystem.
Legal and Legislative Affairs Committee tracks and lobbies for and against legislation, and works with regulatory agencies such as the ATF to represent their member’s interests.
Internationally, accredited United Nations ECOSOC Non-Government Organization (NGO) with Consultative Status. It is their task to be a technical resource for various decision making groups inside the UN.
Conflicting industry standards (C.I.P)
Despite working together, the two main industry standards organizations SAAMI and C.I.P. have assigned different standards for some cartridges.
This leads to officially sanctioned conflicting differences between European and American ammunition and chamber dimensions and maximum allowed chamber pressures.
Some cartridges with possible chamber and ammunition dimensional conflicts, similar to the unsafe combinations listed above, are listed in the Delta L problem article.
Proof test differences
Under SAAMI proof test procedures, for bottlenecked cases the center of the transducer is located .175 in (4.4 mm) behind the shoulder of the case for large diameter (.250 in (6.4 mm)) transducers and .150 in (3.8 mm) for small diameter (.194 in (4.9 mm)) transducers.
For straight cases the center of the transducer is located one-half of the transducer diameter plus .005 in (0.13 mm) behind the base of the seated bullet.
Small transducers are used when the case diameter at the point of measurement is less than .35 in (8.9 mm).
Under C.I.P. proof test standards a drilled case is used and the piezo measuring device (transducer) will be positioned at a distance of 25 mm (0.98 in) from the breech face when the length of the cartridge case permits that, including limits.
When the length of the cartridge case is too short, pressure measurement will take place at a cartridge specific defined shorter distance from the breech face depending on the dimensions of the case.
The difference in the location of the pressure measurement gives different results than the C.I.P. standard
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
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.
Considerations for Magazine Feeding
When a hand loader is working to establish a seating depth to use with a particular bullet, he must decide if he needs the cartridges to feed thru a magazine or not.
If the shooting application is hunting or tactical shooting, then the shooter probably needs the rounds to cycle thru the magazine so the rifle can be used as a repeater.
However, in many slow fire target shooting applications, it’s not necessary to magazine feed the cartridges.
Often times when a shooter doesn’t need to feed rounds thru a magazine, the shooter can take advantage of substantial performance improvements by loading the bullets out long.
This brings up an important reality of seating depth and COAL.
SAAMI COAL Limits Ballistic Performance
It is a fact that the ballistic performance of modern ammunition is directly limited by the SAAMI COAL standards that are currently in place and that rifle manufacturers build to.
Even when a shooter understands the implications of cartridge case volume and has a chamber that allows them to load the rounds out long, the rifle itself (having been built to feed SAAMI length cartridges) won’t allow the shooter to do so.
This fact is one reason for the popularity of custom rifle builders who understand the importance of feeding longer than SAAMI length rounds and building rifles with long enough actions and magazines to cycle the rounds.
The first commercial rifle manufacturers who figure this out and start building rifles capable of feeding longer rounds will lead the way into modern times.
There have been many improvements to several key components of modern rifle ammunition, specifically bullets and powder.
It’s unfortunate that many rifle makers continue to adhere to the antiquated SAAMI limitations that were put in place so long ago when components were so different, standards which limit the performance of modern potential.
Summary of COAL discussion
To recap the important considerations regarding bullet seating depth as it relates to COAL, we can say:
- Seating a long bullet to the restrictive SAAMI COAL can severely decrease the internal volume of the cartridge which will limit the max velocity the cartridge can achieve.
- If magazine feeding is not a requirement (or if you have a longer than standard magazine) you can load your bullets long, which increases the volume for powder and allows you to use more powder and achieve faster MV for the same pressure.
- If you load the bullet too long and it encounters the lands, this can elevate pressure due to the engraving force resisting the bullets’ initial forward motion.
Effects of Cartridge Over All Length (COAL) and Cartridge Base To Ogive (CBTO)
The first half of this article focused on the importance of COAL in terms of SAAMI standards, magazine lengths, etc. There is another measure of length for loaded ammunition which is highly important to precision.
Suppose the bullet was seated out of the case to the point where the base of the bullet’s nose (ogive) just contacted the beginning of the riflings (the lands) when the bolt was closed.
This bullet seating configuration is referred to as touching the lands, or touching the riflings and is a very important measurement to understand for precision handloading.
Due to the complex dynamics of internal ballistics which happen in the blink of an eye, the distance a bullet moves out of the case before it engages the riflings is highly critical to precision potential.
Therefore, in order to systematically optimize the precision of his handloads, it’s critically important that the precision handloader understands how to alter bullet seating depth in relation to the riflings.
Part of the required knowledge is understanding how to accurately and repeatably measure the Cartridge Base To Ogive (CBTO) dimension, and furthermore how to communicate this dimension to other shooters. The following material will shed some light on the subtleties, and pitfalls of the various methods available for measuring CBTO.
Why not use CBTO as a SAAMI standard?
If CBTO is so important to the precision capability of rifles, you might ask, “why is it not listed as the SAAMI spec standard in addition to COAL?”
There is one primary reason why it is not listed in the standard. This is the lack of uniformity in bullet nose shapes and measuring devices used to determine CBTO.
Let’s start by acknowledging the diversity of bullet nose shapes. All noses are essentially a curve that is part of a larger circle. You would think this would make nose shapes fairly consistent. The problem is that the circular arc geometries are different for each bullet design. Even for a given bullet design, tool making is not a precise enough process to make these shapes precisely the same from tool to tool. Add to this the challenge of putting this curve on a surface that is round (like a bullet). Doing this means that the size and location of the curve is influenced by the diameter of the bullet.
When your bullet seater touches the tip of one bullet, the distance to the point on the nose that engages the rifling is fixed. If your bullets have precisely the same nose curve and the same diameter then your CBTO will be very uniform and should easily be able to maintain a +/- .001 tolerance. This is achieved when using good bullets, properly camfered case mouths, and a seater die that does not allow the bullet to bottom out (within the seater die cone) on the tip of the bullet.
Measuring, Recording and Communicating CBTO
There is a vast lack of uniformity in comparators and measuring devices used to determine CBTO.
This is a critical point to understand. To measure from the base of the cartridge to where the bearing surface ends on the bullet you must use a gauge that will attach to your calipers and which also goes over the nose of the bullet to touch the point where the bearing surface transitions into the nose curve.
We already sorted out that bullets can and will vary in this area (at least from type to type if not lot to lot).
This makes it impossible for gauge manufacturers to use one given diameter and shape in there gauges. So there is no standard shape and diameter for gauges. Said differently, gauges can and will vary in both inside diameter and the shape where the gauge contacts the bullet.
There is another reason why these gauges are not standardized. Since bullet nose shapes and diameters will vary, gauge manufacturers know that gauge standardization is impossible.
Since this is true the end result is that this measurement becomes a comparison used by one shooter rather than a consistent dimension used by many shooters.
Given this fact, they are free to open their tolerances up from gauge to gauge. Anyone who understands tooling knows that it is much cheaper to make a tool with a larger tolerance window.
Some of you might be saying, “Hold on a second, if the gauge can vary then how can anyone use CBTO successfully?”
The answers is because since this dimension cannot (or is not) standardized the specific CBTO dimension used by one shooter is critical but this dimension is likely not to match the specific dimensions of a cartridge shot by another shooter.
“Huh?” you say? Let me explain.
If you have one gauge and you are shooting one lot of bullets, you have the ability to measure and adjust CBTO to get the most performance out of your rifle. All of the dimensions using your gauge and bullets are meaningful to your rifle.
Testing to find the best CBTO is a key part of getting the most precision from your rifle and handloads.
For example, suppose that your CBTO using a 308 Winchester is 2.110”.
You take this to the range and it shoots like a house a fire (shoots great). If you call your buddy up and tell him that he should try a CBTO of 2.110” in his rifle he will be grateful until he goes to the range.
When your buddy who has a different rifle/chamber, is using a different bullet (type or lot) and different gauge sets up his cartridge to have a CBTO of 2.110” he will expect the same level of performance. But his rifle doesn’t shoot well at this CBTO dimension. You both are puzzled until you try something.
You take your gauge and your bullets over to his house to find out what he has done wrong. The first thing you do is you measure the CBTO of his ammo.
This is when you find the first problem.
His CBTO is 2.074”. Just as you start to give him a hard time for getting it wrong he pulls out his gauge and measures his ammo again. When he does it with his gauge he gets 2.110”. In this scenario, the difference is due to the fact that your gauges are not the same.
Trying to sort it out further, you decide to load some of your bullets into his cases with his seating die set up exactly the same. Then you should be able to get the same measurement, right?
You load one round and take a measurement. With your bullet at his seater die setting your CBTO is at 2.093”. When he measures this cartridge with his gauge he gets 2.057”. What the heck?
Now you both are all over the place. This second attempt to get things sorted out is thwarted by the fact that the nose shape of your bullets is different than the nose shape of his bullets. You both decide that this is a waste of time since the variation is so much. How can something that varies so much be important to performance?
This simple answer is that you have to apply it correctly and to your rifle using your own gauge and your own bullets.
The first step is to establish the distance from the bolt face to the rifling.
How is this done? There are a two most common ways and neither is without difficulties. The most consistent and accurate way is to load a cartridge purposefully long using medium to light neck tension.
When you chamber the round and close the bolt the bullet gets pushed into the case.
If you slowly open the bolt and remove the cartridge it should be a representation of the distance from your bolt face to where the bearing surface of the bullet engages the rifling.
You need to do this several times because with medium to light neck tension the bullet may pull back out of the neck if it is wedged too tightly into the lead angle of the rifling. If you do this several times and come up with the same dimension (within .001) you can call it good.
There are a few things you need to be aware of when using this method. It is important that you use exactly the same bullet each time.
Not the same type of bullet or same lot but the exact same bullet. If the neck tension is light enough you should not change the shape when you pull it for another measurement.
You also need to measure the COAL to make sure the bullet moved in the first place. You may seat it long thinking that your throat couldn’t be longer than this COAL but find out that when you do this check the bullet doesn’t move at all.
This indicates that either the bullet pulled back out when you opened the bolt or the bullet was not out far enough to touch the rifling. The more you do this check the better you will get at doing it well.
The other common way to get this dimension is to use the Stoney Point (or Hornady) Overall Length Gauge.
This is a device that allows you to push a case into a chamber that holds a bullet in the neck loosely. After the case is inside the chamber you push the bullet forward with a rod until it stops at the rifling.
Standard Chamber Specification
Copyright restrictions concerning intellectual material do not allow distribution of SAMMI and C.I.P. chamber drawings.
However access to their website drawing information and values of the dimensions is not restricted. The published values may be entered into the chamber and cartridge records that are used in this software.
U.S. SAAMI Standards:
Click the link, and when the website opens, select the type of chamber and click on the caliber listing to open the cartridge and chamber data for that caliber.
To convert the inches to metric values, multiply each value by 25.4.
European C.I.P. Standards:
Click the link, and when the webpage opens, go to the bottom right of the chart to select one of several pages of calibers.
Select a caliber and, in the column labeled ”TDCC”, click on ”EN” to view the chamber and cartridge drawings. To view the tolerances and other specs for the selected caliber, click on ”EN” in the column titled ”Annexe”.
The C.I.P. Standard dimensions are listed as metric values.
To convert the metric values to inches, divide each value by 25.4.