Scenario:

-In the process of shooting a full 10-round magazine in my M&P 40c.
-Halfway through the magazine a 'false lock' occurs.  The slide is locked back.  Thumbs were not on the pistol and clear of the slide lock.
-Look down into the chamber, obviously no round in the chamber since the slide is locked back.
-The top .40S&W round is protruding out of the magazine slightly, enough to have hit the magazine follower tab on the slide stop.

What causes this type of failure?

-Weak magazine springs?
-Out of spec slide stop assembly, particularly the tab?
-Worn recoil spring?
-Poor grip?
-Redirecting the recoil of the firearm in an odd manor?  
-Magazine follower tilt?

 
Great problem!  

You've already stated the most probable causes.  The most likely culprit?  

(1)  A worn recoil spring is causing this problem by allowing the slide to travel too fast and, thus, incorrectly picking up the top round.  (It’s all about slide travel, mechanical balance, and timing.)  

(2)  In the alternative:  The slide might be short stroking; and, the question becomes, 'Why'?  Could be your grip; or it could be too stiff a recoil spring for the ammo that's being used.  Might be weak magazine springs, too.  Because the top round on a half-full magazine is, 'nosed forward' it's not the slide stop.  If the slide is either traveling too fast, or short stroking then the pickup rail isn't getting a good grip on the top round's case head, and is catching the top round at its midsection.  This would nose the top round forward WITHOUT properly picking it up.  (Voila!  You've got your problem!)  

While you're diagnosing this problem it's always a good idea to examine, both, the angle of the magazine follower as well as to eliminate any sideways torque in the elongated body of the magazine spring(s).  Just so you know:  The angle of the top coil on magazine spring (The angle that determines the angle on the follower.) usually requires VERY LITTLE adjustment.  In my experience only a slight upward or downward movement is sufficient.  (You should be able to gauge this angle by looking at other properly functioning magazine springs.)  

You’ve just got to try one thing after another until you discover exactly what’s screwing things up.  Now that the most likely causes are known, start here and proceed onward until the problem disappears.  If you want to give yourself the best guarantee for flawless operation, change your magazine springs at the same time you replace the recoil spring.  

(On my semi-auto EDC’s ALL THE SPRINGS get replaced at the same time.  Neither is it uncommon for me to go through 15 or 20 thousand fired rounds without the slightest problem.)  



NOTE:  Yeah, I know it's an older thread; but I like the problem!