Normal load vs higher presssured loads
he P6 came to me with both the Wolff spring and the original braided factory spring. I've changed it out for the factory orignal but have failures of the slide to lock back after the last round is fired even with new mags.
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Perhaps this is why it came with the Wolff spring and the factory original, as my guess is that the factory spring is just a tad too stiff, possibly being why the slide does lock..
You should be able to "feel" the slide banging on the slide stop IF the lighter Wolff spring, using standard 115 or 124gr FMJ (or JHP if they feed) as opposed to a clean, felt, recoil, not being abruptly harsh, per shot.
That was the clue with my Sig P229 9mm, that, at the 4850 fired rounds mark, as the normally smooth, soft recoil, began to have a different feel, as though the gun was pushing +P or +P+ ammo, which it was not.. Changed out the old spring with a new one and the_feeling, harsh impact of slide action ceased..
Keep an ongoing round count, including a rough, already-fired-rounds, and keep in mind that return springs generally need replacing between every 4000 - 5000 rounds from new to old-weak, wearing out.
Luck,
Ls
Ps.. I'm not sure of the Nato 9mm FMJ load, but, IIRC, it was more than our standard 115 or 124gr loads, and being a Euro LEO gun, it may be that the orignal return spring was one designed to work with higher load 9mm type that NATO sanctions..