I've seen it on a few of my new 10 round mags.. The follower did not catch the slide catch lever.. Does it do it on both mags? And yes.. That's a damned pretty sig.
It was happening with both mags. Now that I changed my grip just a tad it's solved, locks back every time!
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I just got my new P226 to the range, and guess what happened after the first couple of magazines... Yep. Altered my grip, and issue resolved.
I've also got a P227, which is wider, and had no such problems bumping the slide lock lever.
Congrats, I love mine!!! Glad it was a simple fix.