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Variables That Affect Ammunition and Rifle Performance

Hopefully, after reading this you will have an appreciation for the many small things that affect the firearm and ammunition combination performance.

Variables That Affect Ammunition and Rifle Performance
The rifle with a barrel shorter than 24 inches will shoot slower than a factory load advertises.

At one time or another we have all been disappointed with the performance of factory ammunition, an off-the-shelf rifle or both. The two most common complaints heard are that the ammunition does not produce the velocities advertised or the ammo and rifle combination is not very accurate.

The typical answer to these comments usually is, “You shouldn’t believe what you read in marketing materials or catalogs.” But are there technical issues at play here? Let’s take a closer look.

As we explore both ammunition and firearms’ mass production techniques and tolerances, we need to understand that when millions of anything are being manufactured, it is impossible to make them all the same. That’s why there are tolerance ranges used around a specification or standard.

Applying this to ammunition, say you set a standard of 2,600 feet per second (fps). Because of the variables involved, you have to allow a tolerance of +/-­ 30 fps. When the manufacturer is loading this ammunition, samples get tested. If rounds test between 2,570 to 2,630 fps, that would be considered acceptable.

The same applies to the groove dimensions of a barrel. If the minimum-­allowable diameter is .3080 inch, then the manufacturer has to allow some maximum larger diameter that they can maintain in production, say .3095 inch. Any barrel’s groove diameter that measures from .3080 to .3095 inch is acceptable.

Going into this discussion, we have to keep in mind that ammunition and the firearm are a system. One affects the other and vice versa. There are certainly manufacturing tolerances in ammunition that affect performance. There are also a number of dimensional tolerances in firearms that have a significant impact on the ammunition performance. Let’s start by examining production issues that can occur with ammunition.

Establishing Standards

Ammunition manufacturers go to extreme lengths to control variables when testing ammunition. This effort dates back to 1926 when the Sporting Arms Ammunition Manufacturers Institute (SAAMI) was organized by ammunition and firearms companies to establish and maintain production standards and specifications for ammunition and firearms.

SAAMI has established test-­barrel dimensions and chambers with extremely tight tolerances. Test barrels are meticulously measured and calibrated with reference ammunition to ensure that testing equipment produces consistent results across the entire industry. If a barrel does not conform to the dimensional specifications or does not produce the correct performance with reference ammunition, it is discarded. Test barrels are retested with reference ammunition at regular intervals and if they are not within the performance specifications, they are removed from service.

As an example of how a very simple thing can affect ammunition performance, let’s look at barrel length. The test barrel length for virtually all SAAMI standardized rifle cartridges, and the barrel length used for advertising rifle ammunition is 24 inches.

Would you expect the Browning X-Bolt Micro with a 20-­inch barrel to produce the same velocity as the 26-­inch barrel on the X-Bolt Hell’s Canyon Long Range of the same caliber? A rifle with a barrel that is shorter than 24 inches will shoot slower than advertised with factory ammunition.

Take a look at Table 1 to see the approximate velocity change for each 1 inch of barrel length for several different cartridges.

In my experience, ammunition from reputable SAAMI member companies is usually very consistent in performance and performs close to published velocities. That is not to say some ammunition can leave the factory with specs that are off. This is why manufacturers test ammunition at regular intervals to ensure performance is held as close to these specs as possible.



Ammo Variables

Ammunition manufacturing involves many variables that have to be dealt with. Projectile diameter can vary by several tenths of a thousandth over a production run of millions of projectiles. Cartridge case capacity, dimensions and hardness can also vary slightly over production runs of millions of pieces.

Look at the SAAMI drawing for the .308 Winchester, for example. SAAMI drawings always show the dimensions for a cartridge with a maximum dimension with a minus tolerance and a minimum dimension chamber with a plus tolerance. You can see that the cartridge dimensions can have fairly large differences in dimensions and still be an in-­spec cartridge.

Of course, whenever these dimensions vary, the performance will differ slightly. Propellant performance can also vary from container to container. Keep in mind that propellant is also mass produced at tens of thousands of pounds at a time, but can only be blended into the final product several thousand pounds at a time. The performance across an entire lot of propellant invariably varies a little.

In my experience, you can see a 30- to 50-fps variation in ammunition because of the variables mentioned above. Manufacturers do their best to control these variables and adjust for them during a production run. If the goal was to load every cartridge to the same performance, little would be produced and no one could afford to purchase it. This is where handloading comes in.

Ammunition manufacturers deal with these issues by establishing performance specifications and tolerances on velocity and pressure for production loading ammunition. They will establish a plus or minus tolerance range, from a standard, that a round of ammunition has to perform within. They will test at regular intervals to ensure that the ammunition performs within this tolerance range. At any test firing that ammunition does not perform within the tolerance range, adjustments will be made. So, what are these tolerances?

SAAMI allows a +/-­ 90 fps tolerance range on rifle ammunition velocities from the standard published velocity. This is a pretty significant range and leads us into tolerance ranges that manufacturers use.

If you are purchasing low-­cost ammunition, a manufacturer will generally use most of the SAAMI +/-­ 90 fps velocity tolerance. It allows them to test less and keep the production machine running a much higher percentage of the time without stopping for adjustments. This lowers the cost.

At the other end of the spectrum, if you purchase high-­cost, premium ammunition, the manufacturer will generally have a much tighter tolerance range on performance to justify the higher cost and higher-­performance expectation. Velocity tolerances on premium or match-­grade ammo is usually in the +/-­ 20 to 40 fps velocity range. That is to say they will load ammunition that will be within +/-­ 20 to 40 fps of their advertised velocity.

Remember, the ammunition could be 20 fps slower than the advertised velocity, but it could also be 20 fps above the advertised velocity. Like I said, nothing can be mass produced by the millions to be exactly the same.

Firearms & Variance

Just like ammunition, it is virtually impossible to mass produce firearms that are all exactly the same and have exactly the same dimensions. As in ammunition, firearm manufacturers have dimensional tolerance ranges that are used during production. Raw materials can vary slightly from lot to lot. Tooling wear, such as wear on a chamber reamer or rifling tooling are a big issue for barrel manufacturers. Lathe or mill accuracy and repeatability must all be considered. These issues (and others) force manufacturers to establish a tolerance range for virtually any dimension on a firearm.

Looking at the SAAMI chamber and cartridge drawing for the .308 Winchester you can see the chamber dimensions and tolerances. There are several of these tolerances that can have a significant effect on ammunition performance. This is why ammunition manufacturers go to such extreme lengths to ensure test barrels have incredibly tight tolerances and test them to make sure they comply. Firearms manufacturers have to accept a little more variation in some of these dimensions or they would produce very few firearms and we wouldn’t be able to afford them.

There are two sets of dimensional tolerances in firearms that have a significant effect on ammunition performance. The first is bore and groove dimensions. The ammunition manufacturers religiously use the SAAMI minimum dimensions for safety and uniformity across the many manufacturers. A firearms or barrel mass producer cannot economically produce barrels that are all made to the absolute minimum bore and groove dimensions. The costs to manufacture and gauge barrels to these minimum dimensions would be astronomical.

This is not to say that mass-­produced barrels are of poor quality or even sloppy; quite the opposite. With recent advancements in machine and production processes, the tolerance on bore and groove dimensions have gotten considerably tighter than 25 years ago. Hammer forging of barrels has been a major improvement in the dimensional quality of mass-­produced barrels.

Typical bore and groove dimension tolerances in mass-­produced barrels is usually about +.001 to .0015 inch above minimum.

Groove diameter also plays a significant role in firearm/ammunition accuracy. Large grooves can allow the projectile to tilt or yaw in the bore. This yaw will be present when the projectile exits the barrel, which degrades a projectile’s accuracy.

To illustrate how groove diameter can impact ammo performance, here’s what I experienced in the early production of 6.5 Creedmoor ammunition and rifles.

What myself and others at the company I was working for encountered were frequent problems with apparent high-­pressure ammunition. Eventually, we got some of the rifles that had problems. The first thing we did was accurately measure the bore and groove dimensions.

In every single problem rifle we checked, the groove dimensions were smaller than the SAAMI minimum dimensions. In some examples, significantly smaller, which caused the projectiles to be sized in the bore.

So, we had a special test barrel made that had groove dimensions of .2625 inch rather than the minimum dimension of .2640 inch. What we discovered was undersized grooves caused substantial pressure and velocity increases.

This phenomenon works in the opposite direction, too. Groove dimensions larger than the minimum dimension cause pressure and velocity decreases. Table 2 shows data from the under-­size groove test barrel.


Other Tolerances

The other dimensional tolerances in a firearm that can have a large effect on ammunition and rifle performance is the chamber reamer and therefore chamber dimensions.

If you look at the SAAMI .308 Winchester drawing, you will see that the chamber dimension tolerances are pretty generous, +.002 inch on diameters and .015 inch on lengths. Firearms manufacturers will use reamers near the maximum dimensions on a new reamer to maximize the life of the reamer and reduce production costs. As the reamer wears and gets dull, they can sharpen it several times and get more life out of the tool and reduce production costs.

If you get a rifle that is near the maximum of the reamer/chamber dimensions, the chamber and throat are going to be rather big. This causes energy loss, swelling of the case, gas blowby around the projectile, increases jump to the rifling and a loss in pressure and velocity.

Gas-­operated, semiautomatic firearms can also cause small losses in ammunition performance. Some of the gas that would otherwise be pushing the projectile is bled off to operate the action.

Revolvers present a whole different set of dimensional tolerance challenges that affect performance. The forcing cone dimensions can allow gas to escape around the bullet. Cylinder gap variations can cause fluctuations in gas lost out of the gap. Small differences in these dimensions can have a significant effect on the performance ammunition will produce.

Hopefully, after reading this you will have an appreciation for the many small things that affect the firearm and ammunition combination performance. The next time you want to dispute marketing and sales numbers as inaccurate, remember all of the things that can affect those nominal specifications.

To all of us that choke when looking at custom gun prices, remember to stop and think why such guns cost so much. Generally, when you buy a custom-­made firearm, you are paying someone for the extra effort to control all the variables discussed above to a much higher level than can be done in mass production. 

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