For those that may be interested, Centerfire is blowing out some LE SW99 .40 magazines -
Used S&W SW99 .40 Cal 12 Round Magazines | Centerfire Systems
Just FYI, back when S&W engineers were trying to figure out why some SW9940's would experience early-slide lock (slide lock with rounds remaining in the mags), they discovered that the magazine body sometimes allowed some lateral movement of rounds rising up toward the top of the magazine during recoil. If the nose of one of the rounds wiggled too far to the left, it would sometimes engage the inside tab of the slide stop, lifting it and locking the slide open.
FWIW, we were told this was also something sometimes reported with early P99's chambered in .40 S&W.
We were told (in SW99/P99 armorer classes) that S&W was working on why it sometimes happened, and studied a range of possible factors. They finally decided they'd figured out the problem.
We were finally told in an armorer recert that S&W had sent the data they'd developed (via high-speed imaging of round stack movement under recoil) to Walther, and Walther worked with their magazine vendor (Mec-Gar?) to address the issue. The result was changing the shape of the magazine body, on the left side, behind the magazine window where the slide stop tab engaged the follower, and the left side of the follower (of course, to conform to the new indentation of the mag body at that spot). The narrowing of the magazine body at that spot was found to prevent excessive lateral wiggle of the rising rounds as they rose up past the slide stop lever's inside tab, preventing them from nudging and lifting the slide stop.
When S&W started receiving the revised magazines, they offered them as replacements for the original .40 mags to their LE customer agencies. I remember hearing they'd also sent them to customers if they reported early slide-lock issues in their SW9940's.
Here's a picture of the two magazine body & follower designs. The original SW9940 mag is on the right side, and the revised magazine is on the left. The revision difference is pretty easy to see on the magazine body. The orange color for S&W models was changed from a light orange to a darker orange to identify the revision. If I remember right, Walther revised their blue .40 follower color, going from light blue to a darker blue.
