I have no clue what it is. A friend of mine bought a house, and the previous owner left a ton of reloading stuff in the garage. She knows that I reload, so she gave it all to me.
So, mixed in with the spent brass, I found about 50 of these. Headstamped 5.56. Most of the inserts are aluminum, but some are white plastic. I thought headspace guage, but I noticed that a lot of them have spent primers, and what looks like powder residue. BTW, they are way to long to chamber in a 5.56.