Guy's, new here, but have a question. I have a Model 66 and 625JM, The 66 I've been shooting for about 3 years, and suddenly I have intermittent light primer strikes, the 625 is about 6 months old, put a longer firing pin in, and a spring set, it was also having intermittent light primer strikes, so I put the original rebound spring back in, but still getting light primer strikes, use only Federal primers in both revolvers, help, this is embarrassing in a match, thanks, Allen

Check the strain screw to make sure it has not backed out. They are meant to be turned in snug.

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Sceva
OGCA SWCA NRA

Since you are shooting reloads, there is a distinct possibility that the primers are not bottomed in the pockets unless you've been messing with the strain screws. Tighten them first like sceva suggests, then examine your primer seating procedure. Federal primers are supposed to be sensitive.

Your spring sets could also be causing your problem if you are using a lighter mainspring. The rebound sling has nothing to do with the striking force of the hammer.

Could be a bunch of causes. A burred FP Bushing, the Cylinder could have severe end-shake, poorly seated Primers, incorrect FP length or overly lightened action. You did not specify if this happens on BOTH SA & DA modes. If only on DA, the Hammer might not be drawn back far enough which might indicate the DA Sear is not proper.

Because you are having similar problems with MULTIPLE revolvers, I'd look closely at the Primers not being seated properly! Federal Primers are not as hard as the CCI's and they are very reliable - maybe the most reliable of all the major manufactures. Other than the "shoe boxes" they come in, they are my favorite Primers.

A thorough cleaning and lubrication of the internals might help.