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3D3DDummy round.
This is a round from some kind of really big gun. It was described as a magnesium practice round, and it has holes drilled in the shell casing that would make it impossible to use in a actual gun, if you had one that big. I'm not clear whether the "magnesium" refers to its being a magnesium incendiary projectile, or that it's made of magnesium because it's a practice round. Neither actually makes a whole lot of sense to me, so hopefully someone will recognize it and tell me more about it one day.

And guess what, reader David Neri helpfully identified exactly what the thing is, and even found three pictures of it.
This diagram from the Federation of American Scientists shows a cross section of the innards:

This picture from the iStockPhoto shows a whole belt of them:

And this picture from the Aircav shows the gun it was used in:

The last source says that these are actually made of aluminum, which seems more sensible than magnesium, and more consistent with the sample I have, which appears to be aluminum. But I never move samples once listed, so this one is still under magnesium. It's quite possibly made of a magnesium-aluminum alloy with a few percent of magnesium, so I'm speculatively listing it as having at least a bit of magnesium. (People often refer to aluminum alloys with a bit of magnesium as "magnesium", for example "mag wheels" are often actually aluminum, though genuine all-magnesium wheels also exist.)

Source: eBay seller chrisredcat
Contributor: Theodore Gray
Acquired: 17 February, 2007
Text Updated: 11 August, 2007
Price: $22
Size: 4"
Purity: <10%