X-Ray Spectrometry of Cartridge Brass
X-Ray Spectrometers Reveal Metal Content of Major Cartridge Brass Brands
Are there significant metallurgical differences in the alloys used in various brands of cartridge brass? The answer is yes, and we have proof. Using a state-of-the-art X-Ray Fluorescence Spectrometer, some tech-savy Wisconsin shooters recently analyzed the alloys in seven different types of cartridge brass.
The test results revealed significant differences in the percentages of copper and zinc in the different brands. Copper content ranged from a low of 72% by mass (Winchester, S&B) to a high of 80% by mass (Remington). Zinc, which adds hardness to the alloy, ranged from a low of 20% by mass (Federal) to a high of 36% (‘brown box’ Lapua). Interestingly, the tests, as reported by Forum Member Fred Bohl, revealed that the alloy in the new ‘blue box’ 6mmBR Lapua brass is different than the alloy in Lapua’s older ‘brown box’ 6mmBR brass. Specifically, the ‘blue box’ 6mmBR brass has more copper and less tin (by mass). Here’s a summary of the X-Ray Fluorescence spectrometry tests:
This testing was done at major science laboratory, using high-grade X-Ray Spectrometry Analyzing equipment. Fred reports that: “The data was run by one of the club members with the permission of the test lab supervisor who is also a club member and shooter. The data in original output reports was far more detailed about trace elements at lower orders of magnitude primarily from surface contaminants (some were rerun after establishing a repeatable cleaning procedure)”. The testing process is discussed in this Shooters’ Forum thread.
We do NOT have the metallurgical expertise to infer that any particular alloy shown above is “better” than another. The alloy “blend” is merely one of many variables that can have an impact on the performance and quality of the finished product. Annealing times/methods differ and some cartridge brass is extruded while other cartridge brass is made with the traditional drawing process. Readers should not presume, on reading the above chart, that they can identify the “best shooting” brass simply based on the constituent metals in the various alloys.
General Observations about Cartridge Brass Alloys
With the cartridge brass X-Ray Spectrometry results in hand, Fred Bohl hoped to find out what “real world” conclusions (if any) we could draw from the raw data. Fred sent the test results to some knowledgeable metallurgists, soliciting their comments. Fred explains: “When I first posted this information [in the Shooters' Forum], I had hoped to elicit replies from expert metallurgists and to initiate a useful discussion. From [their replies] I distilled the following ‘consensus’ comments”:
1. The range of Copper/Zinc ratios suitable for use in cartridge making by typical processes is 85/15 to 65/35 (% by weight or mass).
2. The range of Copper/Zinc ratios suitable for use in cartridges intended for reloading is 80/20 to 70/30. Above 80% copper, the resulting case would tend to be too soft and difficult to attain the distribution of hardness desired (harder at the base and softer at the neck). Below 70% copper the resulting case would tend to be too hard, would work harden too quickly and require frequent annealing. [Editor: That said, the 'brown box' 6mmBR Lapua brass, with 62% copper/36% zinc content, enjoys an unrivaled reputation for both accuracy and its ability to perform well after a dozen or more reloading cycles. We know 30BR shooters who have shot the same old-style Lapua brass (6mmBR parent case) more than 50 times. So maybe the "expert" view needs re-thinking.]
3. As the percentage of zinc increases, the tensile strength, yield strength and hardness tend to increase. However, above 35% zinc, while tensile strength will continue to tend to increase, both yield strength and hardness will tend to begin to decrease.
4. The trace additives of iron and/or silicon are used to control the processing characteristics of the alloy. Trace additions of chromium will improve corrosion resistance and give a shinier surface (both largely cosmetic).
5. Selection of the alloy and additives is a trade off among: end use desired properties; processing time and yield; and cost of materials. For example, the classic 70/30 cartridge brass was considered an optimum combination of corrosion resistance and hardness for single use by the military with good process yield at acceptable material cost.
6. All of my responding experts were surprised by the brown box Lapua alloy except for the oldest. He remembered using an almost identical alloy late in WWII when copper was in very short supply for military small arms ammunition.