It's not exactly the same but it's the closest I can find.

Here's the Wikipedia page on "slungshot", with an illustration. As you can see, it is often misnamed "slingshot", which is different.
http://en.wikipedia.org/wiki/Slungshot
http://en.wikipedia.org/wiki/Slingshot

"Slungshot" (or "slingshot" in the second case) is given as the translation of "rompecabezas" in the following two dictionaries. This doesn't mean it must be right, but I haven't found any other translation for the term elsewhere:
http://www.eudict.com/?lang=engspa&word=slungshot
http://www.spanishdict.com/translate/rompecabezas

The slungshot is a prohibited weapon in Florida. Here is the relevant paragraph:
"(12) "Slungshot" means a small mass of metal, stone, sand or similar material fixed on a flexible handle, strap, or the like, used as a weapon"
https://docs.google.com/viewer?a=v&q=cache:5tPuXAIH3gQJ:www....

It is actually strange that they included the "rompecabezas" in this list, as is pointed out here:

"No es un arma de uso común. De hecho no hay en los repertorios de jurisprudencia sentencias condenatorias por uso de este arma."
http://fboiso.blogspot.com/2007/04/armas-prohibidas.html

Still, it may be the best we can do here.

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Note added at 2 hrs (2012-01-21 15:38:19 GMT)
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This is pretty confusing, and I can't help wondering if the RAE has led us up the garden path here.

Its description of a "rompecabezas" is a short flexible handle with an iron or lead attached to either end. It says "dos bolas [...] sujetas a los extremos", not "al extremo", so there's one at each end. Sounds like a kind of dumbbell.

This is not a description of a "double mace" like the ones in James's pictures, with two spiked metal balls attached by chains to one end of a rigid handle. Obviously the normal word for "mace" in Spanish is "maza", though this "double mace" weapon is really called a "mangual (de dos bolas)" in Spanish.

I have never seen a weapon like the one the RAE describes, and I'm finding it difficult to imagine it, to be honest.

The reference Lucía has found is very interesting, but just leaves me more confused. The Guardia Civil's list mentions "3 rompecabezas", and we can see three objects in the photo that must correspond to this description. The one on the red cloth is undoubtedly a "double mace" like the one in James's pictures, though strictly speaking I think they're flails (ball-and-chain type) rather than maces. The two in the foreground seem to be studded clubs, which could properly be called maces, not double this time. But none of these is anything like the weapon described in the DRAE.

So it looks as though the Guardia Civil is using "rompecabezas" to mean any flail or mace-like weapon, or perhaps any kind of bludgeon. Is this what those who drafted the law meant? Who can say?

Moreover, if the definition of "rompecabezas", at least as applied by the Guardia Civil, is as broad as this, it is very hard to believe that no-one has ever been convicted of assault with a "rompecabezas", as the source I quoted earlier claims.

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Note added at 2 hrs (2012-01-21 15:45:24 GMT)
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That's right, but "slungshot" and "slingshot" are two different things; see the Wikipedia pages I've cited above:

"A slungshot may be swung in a manner similar to that of a flail.
Slungshots were widely used by criminals and street gang members in the 19th Century. They had the advantage of being easy to make, silent, and very effective, particularly against an unsuspecting opponent. This gave them a dubious reputation, similar to that carried by switchblade knives in the 1950s, and they were outlawed in many jurisdictions. [...]
They were also known as "slingshots," but had nothing to do with what is now known as a slingshot."
http://en.wikipedia.org/wiki/Slungshot


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Note added at 2 hrs (2012-01-21 15:47:01 GMT)
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In fact this page is headed: "Not to be confused with slingshot."!

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Note added at 2 hrs (2012-01-21 16:06:19 GMT)
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By the way, the comment I quoted that "no hay en los repertorios de jurisprudencia sentencias condenatorias por uso de este arma [rompecabezas]" comes from an "artículo doctrinal" on criminal law entitled "Aproximación al estudio del delito de tenencia de armas prohibidas", by Luis Belestá Segura, abogado.
http://noticias.juridicas.com/articulos/55-Derecho Penal/200...

He also comments: "Es fácilmente constatable que pocos ciudadanos conocen lo que pueda ser un xiriquete, un rompecabezas o una llave de pugilato. Y menos los que sepan que la tenencia de un tirachinas puede resultar delictiva."

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Note added at 2 hrs (2012-01-21 16:21:53 GMT)
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"En la época prehispánica tenían cuchillos de piedra, arcos y flechas, azagayas (lanzas pequeñas o dardos arrojadizos), rompecabezas ( que eran piedras con puntas talladas que sobresalían algo más de dos centímetros de su forma esférica, y que iban atadas a una rama o tira de cuero de unos 40cm de largo para su manejo)."
http://www.taringa.net/posts/imagenes/808372/Los-Charruas.ht...


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Note added at 3 hrs (2012-01-21 16:41:31 GMT)
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Quite honestly, I think I would be inclined to put "flails and maces" and leave it at that.

I still reckon the weapon that most closely corresponds to the RAE's description is a slungshot. But it's fairly obscure. I had never even heard of it (I had heard of a slingshot, though I've always called it a catapult). As defined in the Florida statute and elsewhere. it's really a kind of flail.

This whole clause on prohibited weapons is very strange. They have gone out of their way to name weapons that practically no one has ever heard of. Did you know what "munchacos" and "xiriquetes" are? They're not even in the DRAE. Even the professionals are confused; I've seen police discussion forums where officers are asking whether anyone knows what "llaves de pugilato" and "tiragomas perfeccionados" are. Quite bizarre.

I think the RAE's definition of "rompecabezas" should just be ignored. Goodness knows where they got it from. It describes a weapon so unusual that there's hardly any point in prohibiting it. And why prohibit this and fail to mention much more common kinds of weapon such as flails, maces, bludgeons, coshes, etc?

It looks as though the Guardia Civil, in applying this law, is taking "rompecabezas" to mean any kind of bludgeon, any weapon for hitting people over the head with. For practical purposes, that seems to be what it means. Never mind whether it has one or two balls; that's a red herring, I think, and only arises because of the RAE's references to "dos bolas". Why on earth would you prohibit flails or maces with two ball but not with one? It doesn't make any sense.

So I would forget about "double ball", and just put "flails, maces or bludgeons", or any combination you feel appropriate.