Revolver timing

Never work on a loaded gun.

Put a light drag on the cylinder's rotation by laying your finger on
its periphery. Cock the revolver slowly. Does the bolt/cylinder stop
drop into the notches in the cylinder before the trigger drops into
the sear notch? If it does you're OK so far. Now do the same thing
but this time cock the gun with the trigger. Does the hammer fall
after the cylinder stop has dropped into the cylinder notch? Good
again. If in either of these tests events happen in reverse sequence,
your timing is off. Either your hand (pawl) is too short or it's too
far away from the axis of the cylinder, generally.

Never work on a loaded gun.

The next thing to do is to check the alignment of the cylinder's chambers
with the bore. Cock the gun and look down the bore at the junction of
cylinder and barrel. You should not see any of the face of the cylinder;
chamber and bore should be concentric. It helps to have a stong light
behind the gun so that light comes through the firing pin (hammer nose)
hole. While looking down the bore, gently attempt to rotate the cylinder.
You should again not see any of the face of the cylinder.

Never work on a loaded gun.

- John Bercovitz (link here)