Was at the range today and the slide on my 9c locked open with one round left in the mag.

I was shooting WWB 115 grain and this batch is not weak. Was also shooting Remington green box and Independence along with Federal Champion.

After this happened I examined everything and found no excessive dirt or issues, gun was clean when I began shooting.

I did not realize I have two different style followers in the mags. I did a search here and verified that these followers are apparently changes made by S&W. Problem is I do not know which magazine was in the gun when the issue occurred. I had no idea that S&W was using different followers.

If you have an idea please post.
Sparx

My 2011 manufactured M&P 9 Pro has the magazines that have the 9 on them. I have 7 magazines now, and they all have the number 9 on the follower, and look just like yours left one - bottom picture.

Ok that is good to know.
When the two followers are compared the one on the right sure has a larger flat area that would be designed to push on the slide stop tab located inside the gun.
I just wish I had noted which mag was in the gun when this occurred but I had no Idea I had two different followers.

Midway sell followers for $1.99, but I am not sure which style they would be.
sparx

The 9-marked magazine followers have been standard for at least two years, my 2009 model has several magazines and all have the mark. (Cartridge case in the case is dated March 2009).

-- Chuck

I have had the same problem with my MP40 on occasion. Did we determine that it is definitely the followers. Both of my mags came with the gun from SW.

Hi David
I got a couple of new followers, I will install them before I fire the gun again.

Since I had the one hold open with a round in mag, never happened again, so that is 100 rounds fired with no issue.

sparx

Just a suggestion. I posted some stuff for another thread here were someones M&P slide did not lock to the rear after the last round. I am a firearms instructor for a federal agency and have seen just about everything. I have also seen what you describe happen due to the shooters grip. A lot of time with a thumb over thumb grip. During recoil the slide stop gets bumped just enough by the bottom thumb that it catches the slide thus locking it back. I have seen it a couple times with the old revolver grip but primarily on SIG's which have there slide stop much farther back. I am not saying there is anything wrong with your grip just some things are quirky if you will. Especially when you use a smaller gun likie a M&P 9C which will recoil a little more than something full size. Just a suggestion to look at.
Good luck

As mentioned, the follower with the 9 on the top is the current one (much like the latest - 3rd - follower for the .40 has ".40" on the top).

While it's often the shooter's thumb that causes an unintentional slide lock with rounds remaining in the mag, it can also be related to the rounds in the mag wiggling sideways (leftward) under recoil and nudging the slide stop lever tab, or even a damaged/weakened slide stop lever spring.

The observation that this is seemingly happening with only the last round remaining in the mag would seem to suggest it might be the bullet nose being knocked sideways just enough to hit the lever's tab, because the mag spring is at its least amount of tension when only 1 round is remaining and can't help tension the last round upward as much as with the previous rounds in the mag load. Simply put, the spring can't overcome the recoil's force jiggling the last round sideways as the last round is being lifted toward the lips during the recoil of the next-to-the-last round.

In the case of a small number of M&P 40c's, S&W was replacing the slide stop levers for this problem with a part that used a heavier wire spring, to better help the lever remain down during the .40's robust recoil. The heavier wire spring is painted yellow instead of light blue, and was reportedly developed during some extended testing of the .357 models. My own 40c had this issue and required the optional lever with a yellow spring (as well as removing a couple of burrs).

That was with the 40c, though ... not the 9c.

I haven't heard of it happening in the lighter recoiling 9c's (as an instructor & armorer), I could see how the shooter (grip), ammo (pressure/recoil force) and the original follower design might allow for this to happen upon some isolated occasion somewhere.

Now, look at the way the new style 9 follower in the previous pics has a change in the left side. This thicker (wider) side of the follower could help keep the last round from moving as far leftward, toward the slide stop lever's tab, under recoil. (This corrective action in follower design was discovered and used in the P99/SW99 .40 mags some years ago (Mec-Gar mags), and apparently the S&W engineers eventually decided to incorporate it in their M&P 40's for much the same reason (also made by Mec-Gar).

If you call S&W and explain you're having some "early lock back" issues with a round remaining in the mag, and have some mags with the older followers, they'll probably offer to send you some new followers to try, or offer to send you a pre-paid shipping label to return the gun so they can inspect it to see if anything else is possibly involved (like a damaged or weakened lever spring).

Just my thoughts.

__________________
Ret LE Firearms inst & armorer

I shoot with one hand, but that does not mean my thumb did not hit the lever.

Smith had the gun back once for failure to extract, and that issue I think has been resolved. I have two new of the latest follower design and will use them in and see how it goes.

The cartridge moving over with the older style follower is a possibility.
TY
sparx

This condition was what S&W discovered was responsible for early slide lock back when the SW99's & P99's were sometimes experiencing it in the .40 models. In that case the mag bodies also had to be redesigned to help prevent leftward lateral bullet nose movement (consider that the .40 has more recoil force acting on the gun than in the 9mm, too).

If it were happening randomly throughout the mag load of rounds, I'd be inclined to look more at the ammunition (bullet nose profile/width), and/or your thumb, but since you report it happening only on the last round, it would seem more likely to be the last round jiggling just enough to the left ... with the mag spring having its least amount of tension with only one round ... and the follower with the thinnest/narrowest left side slope to hold the bullet in the center. The newer follower has more material on the left to move the last round more toward the center, as well as help keep it from shifting leftward as it rises to the lips under recoil.

If the latest followers don't resolve the issue, then it's time to look at the shooter and ammo influence, as well as considering having the slide stop lever's spring checked for shape, strength and overall condition (like at the crimped spot holding it in place on the lever). The factory could simply install a new one if they had any suspicion the one in the gun might be out-of-spec.

Looking at the frame from the rear (slide removed from an empty, field-stripped pistol), from low over the rear, the front end of the wire spring ought to be held to the left under the locking block by fitting within a space molded into the block for this purpose. This is one of those times where it's not necessarily wise to shove the narrow ends of brushes, Q-tips or whatnot up under the locking block to "clean" the frame, to avoid shoving against the wire spring and displacing and/or damaging it at the end under the block (which gives the wire spring its tension to act upon the lever).

__________________
Ret LE Firearms inst & armorer