Did you notice the cylinder whatsits are also just cylinders, with no openings or rounds visible?
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Yup. Even though the hammer obviously strikes what appears to be a firing pin...
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Maybe the entire thing generates some space magic shit and fires a round when it's struck.
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Maybe... and teleports the bullet from the magic cylinder into the magic barrel.
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Imagine that the hammer strikes the cylinder, which works as a drum. That collected force in the drum then discharges a round. Simple space magic. It's the future, man.