Physical Characteristics of Silver Bullets
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The most obvious problem with silver is that it's not as dense as lead. Density is critical to bullet performance. The energy of a bullet after it leaves the barrel is directly related to its mass. Friction (air resistance) eats away at that energy, slowing the bullet as it goes down range. The ideal bullet, therefore, packs a lot of mass in a small volume -- which is the definition of density.
A bit or reasearch shows that while silver is less dense, the difference is only about 7.5%. Furthermore, the lead/tin alloys frequently used to cast bullets are also a few percent less dense than pure lead. Basically, if a silver is fairly 'long' it's cross-sectional density (mass per unit of frontal surface area) should be high enough to give very good ballistic performance.
Before you can pour metal in a mold, you have to melt it. As it turns out, lead melts at a relatively cool 621°F. You can melt lead in an old soup can over a camp fire. Silver, on the other hand, melts at a scorching 1761°F. Your campfire's going to need some help, and your soup can may not fare so well. Also, remember that chemical reactions (like oxidation) happen faster at higher temperatures. The need to flux, or possibly even create an intert atmosphere for the casting is much higher with silver than lead.
There are a number of problems that can occur when pouring hot metal into a cold mold. With lead, the old timers just started pouring and figured that the first few rounds of bullets would be junk, but they'd warm the mold up enough for it to work well. Those first malformed bullets were just thrown back into the melting pot, and everything was good. The problem is that what you're really doing is bringing the mold up to a reasonable fraction of the temperature of the molten metal. Since silver's melting point is much higher, the mold needs to be considerably hotter. As a very rough rule of thumb, the mold should be at least a third of the molten material's temperature to perform well. Getting the mold up to 207°F for lead isn't too tough, but the silver mold needs to be nearly 600°F. Break out the hot-pads grandma, she's getting warm. More accurately, fire up a muffler furnace to pre-heat the mold, and handle it with tongs.
When a metal is plasticly deformed (molded like play dough), it tends to "rebound" a bit. The amount of rebound is different for different metals, and needs to be taken into account when building the molds and dies. If a metal bar is extruded through a hole of exactly 0.5 inch, you won't actually get a 0.5 inch bar, you'll produce something a few thousandths of an inch bigger. Silver rebounds more than lead, in fact quite a bit more. One could make a sizing die that's a little smaller than one designed for lead bullets, mount it in a press that's been modified to handle the higher pressures needed to form silver, and get good results. However, that's a lot of money -- I'm going to try to accomplish the same thing by casting the bullet to the right size (or really, really close), and eliminiating the need for the resizing die to do much of anything.
Actually, in metallurgy, there are several possible definitions of hardness. The one we're interested in is "Resistance of metal to plastic deformation". The harder the metal, the more energy is required to deform it. There are a number of ways to measure hardness, but most reloaders use the Brinell Hardness Number (BHN). Lead is a very soft metal, and has a BHN of 5, silver is much harder, with a BHN of 24.5.
So why is hardness a problem? A bullet is expected to deform in response to pressure a couple of times. First, when the bullet is being loaded, it's run through a sizing die, which insures the bullet is no larger in diameter than it should be. If a cast lead bullet is a thousandth or two oversized, it's pretty easy to re-shape it. A silver bullet is going to take a lot more force, which may break the reloading press.
More importantly, when a gun is fired a pressure wave slams into the bullet, deforming its base and sealing it against the bore. This process is called obturation, and is critical to accuracy. The riflings also etch into the bullet, and begin rotating it. With a silver bullet, the riflings are going to be a little harder to engrave, and the bullet isn't going to deform as easiy. Even worse, a slightly oversize lead bullet will be extruded into the barrel when fired. It would consume some energy, so the bullet may not fly as far or as fast, but nobody's going to get hurt. A silver bullet might not be so accomodating, so we need to absolutely sure they don't exceed the bore diameter. All things being equal, harder bullets tend to consume more energy than soft ones, and show correspondingly lower muzzle velocities. In order to fire safely in an unmodified gun, and engage the riflings properly, a silver bullet needs to be fit to a much higher precision than a lead bullet.
Hardness is not just a problem for silver, it's also a problem for standard bullets. While lead may be soft, many of the alloys commonly used are considerably harder. Hard lead ships better, and is better able to handle things like being loaded into a tubular magazine with a stiff spring. That's good news, because it means that reloaders already have experience in loading and shooting hard materials. In fact, some of the lead alloys being used are very nearly as hard as pure silver. Linotype, for example, is a commonly used "hard lead" alloy, with a BHN of 22. There are calculations showing the minimum pressure needed to properly obturate a bullet of a given hardness, and silver needs about 35,00 psi (242,000 kilopascals). Silver probably won't make a great choice for small-caliber pistols with lower chamber pressures, but for large-caliber hunting rifles this pressure is easily obtainable.
By the way, these hardness numbers do dispell one common silver bullet myth. In many werewolf books, the hero or heroine melts down an old silver coin or piece of jewelry (usually with appropriate sentimental value) to make the bullet that ultimately kills the werewolf. Elemental silver has a BHN of 24.5, but is too soft for jewelry and coins. Those items are built from alloys engineered to make the silver harder and more durable, with typical BHN values between 100 and 150. A BHN of 100 would require chamber pressures of over 140,000 psi to obturate, which is far higher than any handheld firearm is designed for. So, old coins are out, we're working with pure silver!
Everyone who's ever used a thermometer knows that things expand when heated and shrink when cooled. Not too surprisingly, different materials expand and contract at different rates. When you pour a bullet, the material initially forms to the mold cavity then freezes when it drops below the melting point. At that point you have a hot bullet in the mold, which is going to shrink a little bit as it cools down to room temperature. The molds are actually a cut couple of thousandths bigger than the desired bullet size, so that the cooled bullets work out to be the right size. Make sense?
If I cast a .444 Marlin bullet from lead, I want a final diameter of exactly .430 inches. I can calculate that I need to add about 2.4 thousands of an inch to compensate for the lead shrinking when it cools. Silver shrinks quite a bit more than lead. Both its melting point and it's coefficient of thermal expansion are higher. In fact, I'd need to add 11.7 thousands of an inch to the size of my mold cavity. And that, ladies and gentlemen, is why both the MythBusters and Jack Lewis noticed that the silver bullets were smaller than they should be.
Here's what we learned from looking at the physical properties of silver:
- Silver is dense enough to make an acceptable bullet.
- Its high melting point requires better furnaces and technique than is needed for lead.
- Silver coins and old jewelry are too hard, but pure silver (bullion) isn't much harder than commonly used bullets, and should work nicely.
- For proper performance, a silver bullet needs to be cast to higher tolerances than lead
- A standard bullet mold will cast a dramatically undersized bullet in silver, so a custom mold is needed.
Casting silver bullets won't be as easy as I'd hoped, but I haven't found any reason why it can't work. If I manage to overcome the casting difficulties, it should be possible to get ballistic performance on par with hard lead bullets. And that's where this is heading.