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The Army wants a silver bullet. It's not about hunting werewolves or emulating the Lone Ranger, though. The motive, surprisingly enough, is price: Silver bullets should be cheaper than what soldiers are shooting now.

We're not talking about just any bullets here. Rifle bullets still tend to be made of a lead alloy, or these days an eco-friendly copper—silver couldn't compete with those on cost. We're talking about something more exotic. the slugs known as explosively formed penetrators (EFPs), which are fired by some anti-armor weapons.

An explosively formed penetrator starts out as a shallow metal plate or lens, backed by an explosive charge. When the charge detonates, the plate is blasted into a streamlined, aerodynamic slug travelling at high speed over dozens or hundreds of meters.

Perhaps the best-known EFP munition is the thousand-pound CBU-97 Sensor-Fuzed Weapon made by Textron, which bursts over the target area and releases forty puck-shaped Skeet submunitions. Each Skeet descends on an asymmetrical parachute, spinning and surveying the ground below with an infra-red sensor. When a Skeet spots a tank, it detonates, firing an EFP at several thousand feet per second to punch through the tank's thin top armor. It may not completely destroy the tank, but, by targeting the engine, can immobilize it:

[youtube ]https://www.youtube.com/watch?v=bg9uoI8RQKc[/youtube]

In the weapon's debut in 2003, two CBU-97s dropped from a B-52 knocked out the front third of an Iraqi armored column. The unexpected attack was so demoralizing that the survivors surrendered to nearby Marines.

Exotic Metals

While many EFPs are made of copper, in recent years the preferred metal has been a more unusual one, tantalum. Because it is twice as dense as copper, Tantalum makes a much more effective penetrator.

Tantalum is used in the manufacture of capacitors for phones, computers, and other electronics, so it is in high demand. The price can soar over $200 a pound. In addition, shaping Tantalum into an EFP liner is challenging. It has to be made into a disc, flattened in a forge, annealed at high temperature, and then "machine domed" into the right shape. That adds to the cost.

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The Army's search for a more economic alternative led to silver—specifically, a new form of nanostructured silver that is as strong as steel. Metals have a crystalline structure, and the size and arrangement of these crystals affect the strength of the finished article. According to the Army, normal silver has a yield strength of 50-150 Mpa (megapascals), where nanostructured silver will have a yield strength of 250-500 Mpa. Their requirement is for silver with a strength of 300 Mpa. (For comparison, the A36 steel commonly used in the construction industry has a yield strength of 215 Mpa). They know it's possible. A paper published last year by a team at Prague's Institute of Chemical Technology in the Czech Republic describes a new method of producing high-strength nano-crystalline silver by means of "selective leaching combined with spark plasma sintering."

If this project were successful, it could lead to cheaper EFP weapons. At the moment silver costs $224 per pound, in the same ballpark as tantalum. But it's easier to manufacture with silver, which could drive down the overall cost.

New silver EFPs may not be limited to being big, cheaper tank-busters. The baguette-sized Spike missile has been tested with an EFP warhead for taking on light vehicles. The U.S. Army may never need to kill an armored werewolf, but if the occasion arises, they're have the right ammo ready to go.

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