When Callie is about to test fire the revolver, she is shown loading it. She is then shown raising it to a firing position. But when the first bullet is fired, Callie is shown holding and firing an automatic, then she fires a revolver for the remaining 4 rounds.
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When Callie test-fires the revolver found at the SOC, the characteristic tinkling sound of falling cartridges is heard. Revolvers do not eject spent shells - semi-automatic pistols do.