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Cast Bullet Hardness – completing the circle. I got into casting my own bullets just a few of years ago, nearly three decades after I started handloading. For many years, it was not worth casting my own bullets. I could buy high quality bullets from a local caster for very little more than the cost of scrap lead. I could not justify the cost of equipment, not to mention the time needed to cast my own. However, as I got into "odd" or lesser known calibers (41LC, 32-20, 38-40, 44-40, etc), the cost of bullets went up sharply, and in most cases the bullets had to be shipped in, also at high cost. I also learned from bitter experience that the locally available "hard-cast" bullets were not at all suitable for low-pressure CAS cartridges and "soft" bullets cost even more than hard ones. It was time to start casting my own. It has been a very interesting experience. The purpose of this article is to "complete the circle". It shows how to figure how hard the bullet should be for a given pressure load, how to predict what raw material you need to get that hardness, and how to confirm it afterward. There are many articles on parts of this, but nothing I have found relates everything. Originally, I gathered up about 1,200# of "pure" scrap lead, about 250# of monotype, about 100# wheelweights, and a few pounds of pure tin. The "pure" lead (which is now in ingots) was mostly from roofing sheet and tested out at about a Brinell hardness (Bhn) of 5. This means it is not pure in a metallurgical sense (it does not meet "refined" purity standards for lead which has a Bhn of about 4.2), but is about as pure as you are ever going to find as scrap. Originally, I made my own "comparison" type Brinell tester. The LBT tester was temporarily not available at that time. The SAECO tester was expensive and did not read in Brinell numbers. The Lee tester did not exist yet. Mine was a steel ball-bearing sandwiched between a stick of pure plumbers lead I bought from a hardware store (Bhn 5) and whatever was being tested. They were pressed together with a modified "C"-clamp to make an indentation in both pieces of lead. The ratio of the square of the diameter of the two indentations showed what the Bhn of the tested piece was. In other words: Bhn of test lead = 5.0 x (diameter of indent in pure lead / diameter of indent in test lead) ^ 2 That was interesting, but it did not tell me what kind of raw materials I needed to mix in order to get the hardness needed. Testing the hardness after casting is too late. I quickly ran across the formula of what Brinell hardness (Bhn) is needed for different cartridge pressures: Bhn x 1,422 = Cartridge pressure needed to "upset" or "obdurate" the bullet properly. or Bhn = ( Cartridge Pressure / 1,422 ) There are several articles that use slightly different numbers (from 1,400 to 1,450) in the same equation, but 1,422 is a mathematical conversion and it is the correct number. The number comes from converting the pressure in Kg/mm2 (which is what the Bhn is measured in) to Lb/in2 (which is what we use for cartridge pressure). That is: Conversion factor = 25.40 (mm/in) x 25.40 (mm/in) x 2.2046 (lb/kg) = 1,422. There are also several books that say the best accuracy for a plain based lead bullet is when the pressure of the cartridge is 90% to 100% of the strength of the bullet. Two of the best are Veral Smith’s book "Jacketed Performance with Cast Bullets" and Richard Lee’s "Modern Reloading 2nd Ed.", Both cover this relationship great detail. The NRA "Cast Bullets" book by Col. E.H. Harrison and several of the Lyman casting books allude to this relationship by listing what bullet hardness is necessary for best accuracy at different cartridge velocities. They do not mathematically tie the hardness and pressure together, though, as the first two books do. My own tests agree with this theory, at least up to 25,000 to 28,000psi loads. Above that, I have not done any tests and am not sure if the relationship still holds. However, the theory makes a lot of sense. If the cartridge pressure is more than the strength of the bullet, the base will permanently deform when it is fired. The more the cartridge pressure is above the strength of the bullet, the more the bullet base will permanently deform. Everyone knows that deforming a bullet base is bad for accuracy. However, as long as the strength of the bullet is more than the pressure, it will not permanently deform during firing (it will spring back to its original shape after it leaves the bore). OK, it is fairly easy to get cartridge pressure from some of the loading manuals so I could figure backwards to what Bhn I needed. But, what kind of witches brew did I need to mix up to get that Bhn? I found several charts on lead/antimony (Pb/Sb) mixtures vs hardness in metallurgy books. The hardness rises rapidly with small percentages of antimony, but starts to plateau fairly quickly. Increasing the antimony above 20% has virtually no effect on the hardness. There were also some charts on lead/tin (Pb/Sn) mixtures vs hardness. The hardness rises more slowly with increasing tin, but does not start to plateau until much higher percentages than is normally used in casting bullets. In the percentages used by bullet casters (less than 10%), the change in hardness is virtually constant with increases in tin. I never did find a reference chart that related lead/antimony/tin mixtures (L/A/T) vs hardness (Bhn). This is probably because it would have to be three-dimensional. In checking some "known" L/A/T mixtures against their "known" hardness, it looked like there was a complicated relationship between them. I put the word "known" in quotes because identical L/A/T’s sometimes had fairly large differences in Bhn’s reported by different writers. Some people just add the number 5 (the hardness of "pure" lead) plus the percent tin plus the percent antimony for an approximate Bhn. That is: Bhn = 5.0 + (% antimony) + (% tin) That is easy and is also fairly accurate at low percentages of antimony (in the 2% to 6% range), but it gets more and more inaccurate as the percentages goes up. So, with my homemade tester, I melted small amounts of a lot of mixtures and tested them. The rough relationship is shown below. Note that this is an empirical relationship, not a mathematical conversion.
So to develop a bullet mixture, I guess an amount of lead, monotype, wheelweights, and/or tin to melt together. From that starting point, I can figure out the percentages of lead, antimony, and tin for the proposed mixture. There are many places that list the makeup of the materials we melt to make bullets. The percentages vary slightly depending on the article, but this is the general consensus. Monotype = 73.0% Pb, 17.5% Sb, 9.5% Sn, Bhn = 28 For example, if I want to use two pounds of "pure" lead, one pound of monotype, thee-quarters of a pound of linotype, one-half pound of wheelweights, and one ounce of tin, I would do the following calculations (keep in mind that this is more complicated than my usual mix, but it is good for demonstrating the mathematics). Just multiply the percentages shown above by the weight you are using of that metal.
To demonstrate the calculations for linotype in more detail (above) multiply 0.75# x 0.84 = 0.63# of pure lead. 0.75# x 0.12 = 0.090# of antimony. 0.75# x 0.04 = 0.030# of tin. One ounce of tin is 1/16 of a pound = 0.063#. To get a combined L/A/T hardness for that mixture, take the calculated antimony in percent and read the adjacent (partial) Bhn in the chart up above. In the case illustrated, 6.5% of antimony would correspond to a partial Bhn of 11.7 (interpolate, but there is no use in carrying the decimals out to false accuracy). Then directly add the percent of Tin to partial Bhn to get the final calculated Bhn of the mixture. In this case, 4.4% of tin would bring the total to 16.1. This is good for a cartridge pressure of about (16.1 x 1,422) = 22,900psi. To play it safe, you can multiply that by 90% to make sure the bullet is stronger than the cartridge pressure; 22,900psi x 0.90 = 20,600psi. So for best accuracy, the cartridge pressure should be between 20,600psi and 22,900psi with this mix. This is good for a lot of moderate smokeless powder revolver loads. It is too hard for most black powder loads and too soft for Magnum revolver loads. If this is not the hardness needed, I adjust the amount of lead, monotype, wheelweights, and/or tin until I get the hardness desired. For example, the standard 9mm Luger cartridge develops a maximum pressure of about 35,000psi. We need a hardness of (35,000psi / 1,422) = 24.6 if we want to use a plain-base, cast bullet. Actually, you can use a mix anywhere between 24.6 (with bullet strength and cartridge pressure equal) up to 27.3 (with the cartridge pressure 90% of the bullet strength) and still work well. In either case, this is way above anything made from wheelweights, and is hard to get without pure Linotype or even Monotype. That is why gas-checks were invented. For a black powder load in a cap & ball rifle or handgun, the pressure (from what I have read) will run up to about 8,000psi (plus or minus). For that we need a hardness of (8,000psi / 1,422) = 5.6 (or somewhere between 5.6 and 6.2). This would be pure lead with just enough tin to get the mould to fill out. Round ball moulds need very little or no tin in order to fill out while elongated bullets with lube grooves and/or a hollow base often require more tin to fill out. Larger black powder cartridges can develop higher pressures. In those cases, you can see 15,000psi (although some places say up to 18,000psi for large cartridges with heavy bullets). We need a hardness of (15,000psi / 1,422) = 10.5 (or somewhere between about 10.5 and 11.7). For these BP cartridges, straight wheelweights work pretty well (with maybe a little tin added to help get the mould to fill out). This sounds a lot more complicated than it actually is. You quickly get a feel for what is needed and can zero in on a mixture quickly and easily. In addition, Microsoft Excel can be programmed to instantly dump out the results for any combination you put in. In that case, there are no calculations necessary. Naturally, it is best to have approximately 2% of tin to make casting easier. The above example had 4.4%, which is more than enough for "wetting" purposes. If there is antimony, do NOT have more than twice as much antimony as there is of tin. In the above case, the percent antimony is less than twice the percent tin so it is OK. There are two reasons for this "rule-of-thumb". If there is a lot of antimony compared to tin, the final bullet can become brittle. And, it can get more brittle with age. This is not theory. I have had high antimony, low tin bullets that cracked completely through when being roll crimped. It is also difficult to get the mould to "fill out" properly with high percentages of antimony. While tin is known as a "wetting" agent, antimony can be considered (from my experience) as an "anti-wetting" agent. More tin is needed to balance the antimony in order to get the mould to fill out. If a lot of antimony is needed for a hard bullet (for a high pressure load), increase percentage of tin until it is about half the percentage of antimony. I know that the last statement will bother some casters. Tin is expensive and most casters try to use as little as possible, but life is too short for me to try to get a lean tin mixture to behave. Then weigh out and place the calculated lead, monotype, wheelweight, and/or tin into the melter and start casting. I use a small postal scale that goes up to 5 pounds. Afterwards, I test a few bullets to make sure the hardness pretty close to what is needed. I made a few adjustments in the above chart after testing, but it was pretty close first time out. I have since bought a Lee Hardness Tester and repeated the tests run earlier. The Lee tester is inexpensive, fairly easy to use, and accurate, at least as well as I can test it. At about $40, it is also much less than other testers that are available. Its only real problem is that it starts at a Bhn of 8.0. That is too hard for some of my black powder loads. After checking the formulas, I confirmed that the relationships in the Lee chart are purely mathematical. So I extended it downward. A complete chart is as follows:
PS. This chart is exactly the right size to print out, cut out, fold over once, and put in the Lee Hardness Tester storage box. It fits perfectly. I have gotten good results with bullets that I’ve cast using these relationships. Keep in mind that small changes in the Bhn make little changes in the accuracy – with two exceptions; one very big exception and one smaller one. The two exceptions are as follows: (1) Starting with a cartridge pressure that is less than the strength of the bullet, the group sizes will get marginally smaller the closer the pressure gets to the strength. Once the cartridge pressure exceeds the strength of the bullet, the accuracy goes very bad, very quickly. When that happens, the base of the bullet is permanently deformed by the pressure of the powder. The higher that the pressure is above the strength of the bullet material, the worse the accuracy will be. Keep the strength of the bullet slightly above the pressure of the cartridge and you will get the most from your bullets.
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