[Jeff Higgins]

"Hard cast" describes a bullet cast from lead alloyed to increase its hardness. Pure lead is quite soft and only really suited to a few particular applications as a bullet metal. Round balls from muzzle loaders, paper patch bullets, and some groove diameter slugs used in muzzle loaders, such as the Thompson/Center Maxi-Ball. It is soft enough to where it leads the bore very easily even at lower velocities and pressures. I think it runs somewhere around 4-5 on the Brinell Hardness scale.

Tin and antimony are commonly used to harden lead. Sometimes one or the other; sometimes both. Tin has the happy added benefit of making the bigger bullets fill out better in the mold, but is far more expensive than antimony. The addition of either can significantly harden lead, with some mixes getting up into the 20's on the BHN scale.

Common alloys are 10:1 lead/tin, 16:1 lead/tin, and 20:1 lead/tin. Antimony is typically used in far smaller quantities, more like 2%-5%, but still results in harder bullets. A 10:1 lead/tin mix (if I remember correctly) will yield a BHN of somewhere around 12-15 or so, where about 3% antimony will do the same.

Another way to harden lead bullets is to heat treat them after casting or to quench them in water as they drop from the mold. Either method is dependant upon having antimony in the mix; tin does not respond to either method. Such heat treating can actually get the BHN up into the 30's. A lot of high power cast bullet shooters do this in their target rifles. Interestingly, they will degrade over time and return to their original hardness. They will also lose hardness when passed through a sizing die.

For the rest of us, BHN's over the low 20's are pretty useless; the bullets become quite brittle. BHN's in the 10-15 range have proven the most usefull for all-around shooting in most handguns and BPCR's, where the vast majority of cast bullets are used these days. They work great for hunting, and are just soft enough to obturate to fill the bore when fired.

So there you go. "Hard cast" is one of those old terms that no one really knows the origin of; gramatically challenged, but descriptive. Kind of a mainstay of the shooting world. Rather general, as it covers a broad range of hardnesses, but most folks know what you are getting at. The details can follow from there.