S&W 625 PC fire pin

I'm going to ask for some help for a friends revolver that gives light primer hits when using double action.
We shot some practice the other day and it wouldn't set off 2 out of 5  then 3 out of 5, worked better when single action, but he shoots all sustained double action. 
  I looked it over and the strain screw is tight, so I had a new ribbed mainspring and that didn't help, I pulled the trigger and looked at the firepin distance out of the recoil shield, didn't measure it but it looked pretty good. But I can't figure the light primer strikes, the gun is clean and he just totally cleaned it when outdoor season ended. This isn't his first time with this problem, he put in a extended firepin in it before. I'm thinking, 1: maybe the hammer is dragging inside sideplate or 2: maybe that floating firepin isn't sliding forward far enough, or headspace is excessive (unlikely), he uses moonclips with the 45acp cases. Iv'e never looked at the new sliding type striker pin or what ever they call them yet. I also don't know the specs on these newer revolvers compared to older N frames.
  I'm still thinking how they work, hammer contacts pin, pin slides forward hits primer, boom. Does hammer stay with pin all the way to primer or is it like inertia of pin alone that hits primer. 
  Stan

Wasn’t there a guy on here recently that had some too thin clips..?

My model 14 won't reliably set off CCI primers in double action, but is 100% reliable with S&B primers. Factory mainspring. I'm sure the gun can be made reliable by tuning it, but changing primer brand may be easier.

What parts have been replaced? Have all the factory parts been returned to the revolver or are any replacement parts still in the firearm?

Why did he/she replace the factory FP with an extended firing pin? What problem was that new part supposed to fix?

Colt DA revolvers have used that system for decades and it works well.

Wobbley wrote:Wasn’t there a guy on here recently that had some too thin clips..?

Aftermarket moon clips [1/3rd, 1/2, or full] are sometimes problematic and have been for years. I had a S&W model 25-2 [.45 ACP] that ran perfectly with factory 1/2 moon clips and then intermittently with aftermarket clips. If the clips are too thin, too thick, or warped, they'll cause issues.

You might check the length of the strain screw to make sure it hasn't been shortened.

Federal primers are the softest (easiest to ignite).  The PPC guys established decades ago that for critically tuned revolvers Fed primers were the standout most reliable.  That doesn't address the "why?" part of your post, but it may be a solution for you.