Ask ten bullet casters what alloy they swear by and you will probably get ten different answers, along with strong opinions about why everyone else is wrong. That is part of the fun of this hobby and trade. But underneath all the personal preference, there is real metallurgy driving those choices, and understanding it will save you a lot of trial and error at the mold.
Why Alloy Choice Matters More Than People Think
When you cast bullets, you are not just pouring melted lead into a mold and calling it done. The alloy you choose determines how the bullet performs at the exact moment it matters most: when it is under massive pressure, spinning through rifling, and dealing with friction and heat all at once. Get the alloy wrong and you get leading in the bore, poor accuracy, or bullets that deform before they even leave the barrel.
The three big levers you are pulling are lead purity, antimony content, and tin content. Each one changes hardness, fluidity, and how the bullet behaves under pressure.
Matching Hardness to Your Load
This is the question we get asked constantly: how hard does my bullet alloy need to be? The honest answer is that it depends on your velocity and chamber pressure, not on some universal “best” hardness number.
For low velocity target loads and cowboy action shooting, softer alloys in the 8 to 12 BHN range often perform beautifully. Soft lead obturates, meaning it expands slightly to seal the bore, which actually improves accuracy at lower pressures. Going too hard here can cause the bullet to skate over the rifling instead of gripping it.
For mid velocity handgun loads and general rifle plinking, alloys in the 12 to 16 BHN range are a common sweet spot. This is usually where a Pb-Sb-Sn blend with a modest antimony percentage earns its keep, giving you enough hardness to resist deformation without sacrificing the fluidity you need for clean, fully filled-out bullets.
For high velocity loads, magnum handgun rounds, or high pressure rifle cartridges, you often need harder alloys in the high teens to low twenties BHN. Here, higher antimony content becomes important, sometimes combined with heat treating the cast bullets to push hardness even further. Softer alloys at these pressures tend to lead the bore badly and lose accuracy fast.
There is a rough rule of thumb some casters use, matching bullet hardness to chamber pressure in a roughly linear way, but treat it as a starting point, not gospel. Your specific gun, bullet design, and lubricant all play a role too.
What Tin Actually Does for Casters
Tin gets a lot of love in bullet casting circles, and for good reason. It is not primarily there for hardness. Its real job is improving fluidity, helping the molten alloy flow into every corner of your mold so you get crisp, fully formed bullets without rounded edges or incomplete fill-out. A little tin, often in the 1 to 2 percent range, can turn a frustrating casting session with frosty, incomplete bullets into a smooth run of keepers.
Tin also improves the alloy’s resistance to oxidation while molten, which means less dross and a cleaner pour. If you have ever fought with a batch of alloy that just would not fill your mold properly, tin content is usually the first thing worth checking.
Antimony: Your Hardness Dial
Antimony is the primary hardening agent in most bullet casting alloys. It increases hardness, improves as-cast fluidity in its own right, and helps the bullet resist deformation under pressure. The tradeoff, as always, is brittleness. Push antimony too high without balancing it with tin and you risk bullets that crack or shatter rather than expand or hold together, especially in hunting applications where controlled expansion matters.
Building a Custom Alloy for Your Application
This is exactly why we started offering custom bullet alloy billets. Off-the-shelf alloys are a fine starting point, but serious casters know that dialing in the antimony and tin ratio for their specific load, mold, and velocity range makes a real difference downrange. Whether you are casting for cowboy action loads, magnum handgun rounds, or precision rifle work, we can formulate a Pb-Sb-Sn billet matched to your hardness target and fluidity needs.
If you are just getting into casting your own bullets, start by figuring out your velocity range and working backward to a hardness target. From there, alloy selection stops being guesswork and starts being a repeatable process. And if you would rather skip the trial and error entirely, reach out and we will help you build a custom blend suited to exactly what you are loading.