As a preface to an upcoming series on subsonic rifles, I have compiled the following information regarding metals for bullets.
To produce good ballistics a bullet needs several characteristics.
- Internal: It has to remain solid at the temperature and pressures of firing, to avoid melting in the gun. It also must be softer than the barrel through which it is fired so that it conforms to the rifling of the barrel (which is critical to ballistic stability) and so that the barrel can be shot repeatedly without degrading.
- External: It should be as dense as possible, since density not only increases stability but also reduces the energy lost to air resistance during flight.
- Terminal: It needs to be tailored for some terminal objective. Depending on the target, we may want a bullet to explode, expand, penetrate, stop, or do some combination of those things.
Following is a list of elements that are of interest in meeting these objectives, ordered by density and noting their rough current cost:
Internal ballistics: Not too hard, not too soft
Lead, being a very cheap and dense metal, was the starting point for bullets. In order to harden it and make it easier to cast, lead is typically alloyed with up to 20% antimony and 10% tin. In higher-pressure modern rifles it is often necessary to crimp a thin copper “gas check” to the back of the bullet to prevent its base from melting or vaporizing in the barrel.
As bullets are driven faster and harder it becomes necessary not only to shield the base with copper, but also to surround the sides that engage a barrel’s rifling with copper. Typical copper jackets are composed of brass or “gilding metal” — alloys containing a small amount of zinc. Brass has a higher melting point and lower coefficient of friction than lead, so it reduces metal build-up and wear in barrels. With a copper jacket the bullet’s lead core can be reduced to relatively pure, dense, and inexpensive lead. These benefits have made copper-jacketed, lead-core bullets the standard for modern firearms.
External ballistics: Looking for an edge in Density
Long-range shooters obsess over the ballistic coefficients of bullets. The higher the ballistic coefficient the less a bullet is slowed by air resistance and the further its effective range. Given a gun and bullet of a particular size, you can first increase ballistic coefficient by streamlining bullet shape. But after that the only way you can improve external ballistics is by increasing the bullet’s density.
Included in the table above are the densest seven elements that are not dangerously radioactive. (Uranium 238, known as “depleted uranium,” is the isotope we consider here. It is radioactive, but its half-life is so long that it is not considered a radiation hazard. Its toxicity is similar to lead.) The next-most-dense metal, tantalum, is less than 50% denser than lead, and hence not worth considering. Unfortunately most of these densest metals are also among the rarest, as you can see by their cost. We’re not much more likely to see people shooting osmium-core bullets than gold-core bullets, even though at twice the density of lead osmium would double the ballistic coefficient!
Two of the densest metals are available at prices that don’t immediately disqualify them from consideration: Uranium and tungsten. These days people will pay as much as half a dollar for a match-grade lead-core target bullet, or a dollar for a premium hunting bullet. It’s plausible that they will pay a few multiples of that for a bullet that’s significantly more dense. But each of these two metals has some quirks beyond cost that limit their availability in ammunition.
Tungsten is extremely difficult to work in solid form. It has a very high melting point and is both very abrasive and very brittle. Solid alloys have been used to produce armor-piercing bullets, but their density is reduced to no more than 15g/cc while their manufacturing costs far exceed those of comparable uranium penetrators. Pressed tungsten powder cores are much easier to produce, and are used in a few frangible bullets currently on the market. But powder metallurgy keeps their density under 12g/cc — not much better than lead bullets.
Depleted uranium is soft and melts at low temperatures. In fact it behaves similarly to lead with one key exception: Its affinity for oxidizing (i.e., burning). So long as it is done in an atmosphere purged of oxygen it can be easily cast into bullet cores. After jacketing a uranium-core bullet is as safe to handle and shoot as a lead-core bullet. In practice uranium offers the only practical and cost-effective means of improving on the density of lead bullets: Finished uranium-core bullets can be up to 70% denser.
Uranium cores have some unique terminal applications. Since uranium is pyrophoric uranium-core bullets tend to be incendiary. Pure uranium is malleable, but when alloyed with small amounts of titanium it becomes very hard, and even develops a “self-sharpening” tendency to fragment into shards that makes it an ideal penetrator. Given these characteristics it’s no wonder militaries have adopted uranium penetrators for anti-armor projectiles.
Conventional incendiary bullets are created by supplementing the core with titanium, zirconium, or other similar metals that oxidize easily and burn hotly. The thin jacket ruptures on impact with a target and the heat of impact is enough to ignite incendiary payloads.
In general a bullet’s ultimate goal is to deliver energy to a target, and as we already established denser bullets do this more efficiently. Exactly how the bullet dumps its kinetic energy into a target is a subject of many debates and a plethora of designs. Hunting bullets are tailored to do everything from fragmenting immediately on impact to expanding to a controlled size and then penetrating as deeply as possible without breaking up.