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I found these at the range today. The shoulder is some sort of plastic material, and the web/head is like some type of insert that can slide up and down. I can't physically slide them down, but they will move. I did find one that was slid all the way into the case. If it helps any they're made out of aluminum. I haven't stuck my dial calipers to them yet but visually they seem like they are 223/5.56.
