It is basically a designer's choice. When designing a revolver, there is little leeway about what parts are needed - a hand, a cylinder stop, a hammer, etc. When the designer figures out how to get all those in the way he wants, the direction the cylinder turns may well be dictated by how the other parts fit together. For example, if the only place he can put the hand is on the right, the cylinder will turn counter-clockwise, and vice versa.
Jim
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