G.T. once assisted me with a '38 Mauser that would not reliably hold open. We improved performance by taking a couple coils off the mainspring, but this did not 100% cure the issue. G.T. then found a Mec Gar magazine with a very slightly larger follower button than typical. That magazine fixed it.
But, after a few months, the issue returned. Experimentation showed that a stronger magazine spring in that one magazine would again cause the hold-open to work reliably 100% of the time, so G.T. changed to a stronger spring. For the last couple years, the hold open has worked reliably with this stronger spring in the one magazine. Use of any other magazine, including Mec Gars with a stronger spring, and the hold-open does not work reliability.
I have found that measuring the height of the hold-open above the frame with an empty mag inserted and the upper removed, is a useful indicator of how the hold-open will perform. With the hold open clearly projecting above the frame, it works. When it sits flush with or slightly above/below the frame, it does not work.
As you might imagine, G.T. and I sent numerous parts and the frame back and forth several times in pursuing the fix(es). It was actually fun to work through the issues and quite satisfying when we got the pistol to function reliably.
For me at least the moral of the story is hold open issues can be a bit of a challenge and may have multiple causes and fixes. Hope this helps, good luck.
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