Lead Bullet Hardness
The Importance Of Bullet Hardness
The hardness of the lead alloy used to make cast lead bullets should be controlled to produce optimal performance depending on the firearm that will shoot them. Muzzle loaders should shoot soft lead bullets to allow the bullet to deform slightly when loaded by pushing it backwards down the bore and to expand into the rifling for optimal performance. Pistol bullets should be harder, magnum pistol bullets should be even harder and rifle bullets should be the hardest. Even then, rifles usually use lead bullets with gas checks, which are shallow copper cups pressed onto the back of the bullet to prevent the hot combustion gases from eroding the base of the bullet or blowing past the bullet. When the lead bullet melts, some lead will be deposited in the bore. This reduces accuracy, and in extreme cases can result in dangerously high pressures as the bore diameter is decreased by the lead fouling.
Contrary to popular belief, harder lead bullets are not always better. The bullet hardness for a particular application is determined by the chamber pressure. The harder the bullet, the higher its yield strength. If a bullet is too soft for the chamber pressure, the hot gases will squeeze past the bullet. The bullet isn't hard enough to plug the barrel and resist the higher chamber pressure. However, if a bullet is too hard, it won't deform into the rifling, which will result in small leaks of hot gas through the rifling, creating a similar problem with lead depositing in the bore. Obturation is the process of a bullet expanding into the rifling under the pressure of the expanding gases. A bullet that is too soft or too hard for the chamber pressure will not obturate, resulting in excessive bore leading and decreased accuracy.
Fortunately, there is a fairly large range of bullet hardness that will avoid bore leading, so only a rudimentary understanding and minimal control over the process is needed to safely shoot lead bullets in most pistols. Often, it's as simple as melting wheel weight lead and casting bullets, or casting them into a water bath to increase the hardness.
Lead bullets in rifles are actually very accurate, but more careful attention to lead hardness is needed to produce the best possible accuracy. The best accuracy usually occurs when the chamber pressure is slightly less than the bullet yield strength. This allows the bullet to fully seal the bore by expanding into the rifling, without being so soft that the hot gases are able to squeeze the lead out of the way and sneak past the bullet. For a complete understanding of lead bullet hardness, please refer to Modern Reloading, Second Edition by Richard Lee, chapters 8 through 10.
Controlling Lead Bullet Hardness
Pure lead is often described as "dead soft". For all practical purposes, pure lead can't be hardened. To increase the hardness, other metals are added to lead to form a lead alloy. Adding tin will make lead slightly harder, but a lot of tin must be added to significantly increase the hardness, and this has two adverse effects. The bullets become too expensive and too light. Antimony and arsenic are more common and more effective alloying agents to increase lead hardness. A much smaller amount of these alloying agents is needed to harden lead, and the density is not lowered much by these small amounts.
When antimony is added to lead, the alloy can be hardened. In most lead alloys, arsenic serves only as a catalyst in the antimony hardening process. Despite its ineffectiveness as a hardening agent, a small amount of tin is beneficial in casting lead. Tin greatly reduces the surface tension of lead, allowing it to fill a bullet mold more easily, resulting in sharp lines and edges.
A common source of casting lead alloy is scrap wheel weight lead recovered from tire shops. The composition of wheel weight lead alloy varies, but is typically about 95% lead, 4.5% antimony, .4% tin and .1% arsenic and other trace elements.
One simple hardening method is quench cooling. This involves quickly cooling the newly cast bullets, and this is usually done by dropping the bullets directly from the mold into a cold water bath. A more effective but more involved method is oven curing, where the bullets are baked for one hour and then immediately quench cooled in a water bath.
Typical Wheel Weight Bullet Hardness
Air Cooled 12
Cast bullets made from alloys including antimony and/or arsenic that are air cooled will continue to slowly harden for years. Bullets made from the same material that were quench cooled or oven heat treated will slowly soften for years. This is called "age hardening" or "age softening".
Lead will work soften. Mechanical operations on lead alloys will result in the lead becoming softer in the effected areas. After casting bullets, they should be processed through a sizing die with no lubrication prior to oven heat treating, and then they should be lubricated. Sizing after any heat treating process will result in softened drive bands. Much of the surface hardness achieved by quench cooling bullets will be lost if they are resized, and an oven heat treatment after resizing will be required to produce hardened drive bands on the bullet's shank.
More detailed information can be found at the following online resources.
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