I have problems with my 625-4 when firing 45 ACP rounds with moon clips.I have misfires from time to time but 100% ignition with 45AR. Has anyone tried the longer nose Power Custom offers for 100% ignition ?
With the hammer nose failure to fire, the first place to check is the main spring strain screw. Make sure it is screwed down tight to the frame. I don't use moon clips, but it occurs to me there is various thicknesses, you you might measure yours to see how thick they are. There will most likely be someone along here who knows all about moon clips!
What nightowl said, all moon clips aren't the same, they are different thicknesses depending on maker. Try finding some that are thicker instead of changing the gun.