Hi All,
I took my new to me 64-5 to the Forest today and fired a bunch of reloads. I had some ignition problems on about half of the rounds I fired in double action mode. No issue with single action. Can you please suggest the likely cause? I figure I could get a Wolff spring pack and just replace them all. I also have a bunch of old model 10 parts that would probably be good enough to solve the problem. I sure appreciate any advice. Thank you.

After a good cleaning, I would check the mainspring strain screw, the mainspring, and also, double check for end shake. Too much headspace can lead to ignition problems. (rear gauge, between the rear of the cyl and the breechface should be .060" - .068". (smaller is better)

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Ret. LE, FA Instr, S&W Armorer

My first guess is the strain screw has backed out or was loosed to achieve a light pull. Second thought, someone filed the strain screw to a shorter length to achieve a light pull. Third, aftermarket main spring.
Another possibility, with this revolver being new to you, you have no idea what sort of maintenance it received during its life. The action could be a little gummy with old, dried lubricant and possibly even WD-40.

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VCDL, GOA, NRA

Another possibility is that the primers in your reloads weren't fully seated.