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The chamber roughness could indeed be adding to hard extraction. However I tend to think your main problem is from the shape of the pins themselves. Even with perfectly timed cocking levers a slight movement of the bbls is necessary to start the hammers back. With non-rebounding hammers in then becomes necessary the shape of the pin nose allows it to cam back slightly with that first motion, & not tend to "Lock into" the indent. This is usually best accomplished by a full hemi-spherical shape to the nose.
