Could be ingress of dirt/powder residue in the firing pin holes. There's a sticky about removing/changing firing pins in the Browning sub forum. Probably a good idea to replace the pins at the same time for the sake of not much money.
Blair, any updates? My late 60s Daly Miroku recently developed the same problem. I'm about to tear it down as much as I can and clean it. But if I need a part that may be a problem. The problem for Daly Miroku shooters in the US is that all Mirokus are sold through Browning and we can't get new parts for pre-Browning Mirokus.
I had this same problem on a 1969 20 gauge Miroku, cleaning did not help and I had to have a new hammer spring installed. There is a gun smith in AZ that still has new old stock springs for 12 and 20 gauge guns.
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