I apologize in advance for the length of this post. Hopefully it will make a bit of sense.

I've spent a bit of time at the range this weekend with both my 40 FS and a new 40C. Based on my testing, I now think the hesitation that I've experienced and has also been reported above may be due to the slide stop moving (bouncing) under the forces of recoil and trying to catch in the notch in the slide. This causes the hesitation felt as the slide goes forward. Sometimes it just briefly catches the notch, sometimes it will cause the slide to lock back as if the mag was empty.

The condition was only noted when shooting full power loads and as the mag was partly empty. The partly empty mag provides less pressure on the underside of the slide and a faster slide velocity.

The first time out with the 40C yesterday, the slide locked back early several times when firing full power 180 grain factory loads and heavy handloads. Especially when, as noted above, there were 5 or fewer rounds left in the mag. Light handloads and Privi factory loads (very mild!) ran without problems. BTW these light loads also cycled properly in my 40 FS with a 20-lb ISMI spring installed.

There were no marks on the slide stop to indicate that it was being prematurely actuated by a round in the magazine.

I loaded up some "medium-heavy" 180 handloads and tried them this morning. These loads did not lock the slide early, but I did get the "jerky" slide return, as seen before, after the mag was partially emptied. This was also noted, to a lesser extent with the FS.

If anyone having this problem has access to some lighter loads or handloads, try some and see if your pistol runs smoothly.

The ideal solution would be a heavier slide stop spring, but I am not sure if one is currently available. In the meantime, I'm going to have to buy a spare slide stop assembly to play with and see if I'm on the right track.