The square hole in the hammer is tapered. It is designed to lightly press onto the tumbler shaft. If I is loose, it could be wear, or a sign that the hammer is mismatched or tumbler replaced from another gun.
A way to tighten this up is to remove the hammer and lightly strike the hammer surface on the 4 sides of the square hole. Or with a pin punch large enough to span over the whole square hole as to get all sides at once. But you wan to make sure you are well under the screw.
Basically you are lightly knocking the metal down to make that taper grab the shaft tightly.
This is the best way I know of to remedy this issue without more extensive modification to other parts.
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B. Dudley
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Threaded Mode