ID Help Please

A friend gave me this and had no idea what it is Lth 2.168 top Dia 0.243 outer case dia tapers from at the bottom to shoulder 0.366 --o.350 inner case 0.527 rim 0.371

Markings below shoulder UTM 5.56 RH by the rim 18 and on the brass on the bottom it looks like 3 diamond shapes with the outer 2 over lapping on to the center one

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Wow that was quick thank you very much