Small brass.

[bartshop62]

What is this? found it in ww2 military base? How old is it? Is it military?

 

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[ToddsPoint]

Appears to be a semi wad cutter bullet of some sort. I can see the grooves on the side from the rifleing. Gary

 

[Tony in SC]

It is a bullet. Not brass but jacketed. Accurate dia. measurement will give the caliber.

 

[ecmo]

Using a caliper to measure it instead of a ruler is the way to determine caliber. By your approximation measurement I would say a hollow base semi wadcutter in .45 caliber. See if with the use of a caliper if it measures in the .452"-.454" range.