View Full Version : M&P .40; Magazine Springs Wear Out?

April 26, 2012, 02:31 PM
On my last few outings, my M&P has failed to lock open on the last round fired. Last night I tore everything down (way beyond field strip; down to the last pin) and cleaned to make sure the slide release lever moved freely, no gunk anywhere (or over-lubed) in the trigger group. Tore down the mags and cleaned them out for the same reasons.

Still, there's a 50/50 shot of the slide locking back on an empty mag when you rack the slide. When I remove the slide and insert a mag, the slide release pops up fully as it should. However, when I push the magazine follower down into the mag and then release it, the slide release only comes up partially most of the time.

Are the mag springs getting weak? This pistol stays loaded, and I'd say that both of the mags that I have, have gotten the same amount of time being loaded for long durations. I can't think of another reason for this to crop up after about three years of service.

James K
April 26, 2012, 06:58 PM
There are other possible problem areas, but what you can do is buy some new mag springs, either OEM or from a spring company like Wolff. If that is NOT the problem, you won't have spent a lot of money; if it is, your problem is solved.


April 26, 2012, 07:29 PM
Thanks for that, Jim. Wolff doesn't seem to have anything for the M&Ps, but Brownell's has them for $7.99 a piece. I'll order a few.

If that's not it.... then what? The only other thing I've really noted is some additional wear on the actual slide release sear... but it's very minor. Everything else seems to be as it should.

April 26, 2012, 10:06 PM
With an empty gun lock the slide open and look down into the magwell and see how much engagement you have (or not) between the follower and the slide stop's lug. Manufacturing tolerances can vary between the people who make the guns and the people who make the magazines to the point where the slide stop cannot reach the follower. On some magazines the follower can be "adjusted" (bent) enough to make it reach. It sounds like your followers have a lot of "wiggle room" inside the mag. Personally I wouldn't sweat it too much. All of my pistols have been modified NOT to ever lock open on empty because I have seen too many instances where a pistol that locks open on empty will also lock open halfway through a magazine. I have learned to count the shots and never shoot the gun dry. This is how most IPSC competitors do it. (probably not legal for IDPA) If your not having any failures to feed the last round with any magazines then your mag springs are OK. As soon as any last round feed problems show up than your mag spring is toast. On high cap double stack mags I would download the mag by 2 or 3 rounds IF you're going to leave it loaded full time. And if your mag springs are 3 years old they're most likely shot by now (especially if they have been loaded to full capacity most of their life.)

May 7, 2012, 09:58 PM
I've got plenty of engagement between the follower and the lug. For gits and shiggles, I took out one of the mag springs and over-stretched it, put it back in the mag, and the problem went away. Definitely the mag springs wearing out on me.

James K
May 8, 2012, 02:13 PM
Just curious, but are the magazines S&W factory?


May 9, 2012, 12:17 PM
Yip. The two that came with it. I thought about calling them to see about a swap out... Haven't remembered to do it at an opportune time though.

May 9, 2012, 01:35 PM
"Stretching" a worn spring will pretty much destroy any temper it had left. Buy Wolff extra power springs (5%) or ISMI. Cheap springs don't last long. When I shot competitively I replaced all my mag springs at least twice a year.

James K
May 10, 2012, 08:02 PM
You might try removing the slide stop and making sure it is smooth where it contacts the magazine follower; it has to allow the follower to move just a tiny bit when it engages. Also, make sure the followers are smooth and not picking up crud, and that the inside of the mags is clean and dry; if you want to lube them, use a dry lubricant, not oil or grease.


May 10, 2012, 09:19 PM
Good advice from James K.

May 30, 2012, 03:32 PM
Ok now I know that this is for the M&P .45 however I had a problem where my magazines were literally feeding 4 rounds and then the springs were bound up and would not feed anymore. I took them apart cleaned them and tried again and the same thing happened. I called SW on this matter and they sent me new magazines and had me send the old ones back. Then I had a package show up on my door the next week with new springs in it for the remaining magazines I had left. They also included a letter saying they had redesigned the spring and were sorry for the problems. You might try calling them on it. They may know there is a issue and take care of it for you. I will note that as it was a rather smooth dealing, the first person I talked to told me I didnt know how to properly load a magazine and that was the issue , I asked where they would like my police certification sent to saying I could handle a firearm and load it.Then the whole attitude changed. :D

May 30, 2012, 05:41 PM
Magazine springs can become weakened and worn over time. The company recommends that its LE armorers replace the mag & recoil springs either every 5K rounds fired, or every 5 years of use (since LE users typically leave their magazines loaded all the time).

So, how long have you owned your M&P40, and how old are the mags?

Also, there have been a couple of revisions of the .40 followers since the model was introduced in Jan '06.

Here are the .40 followers, running from the original/oldest on the left, up to the current/newest on the right (with the number 40 on the top). Notice how they've changed the profile to keep the last round better centered, as well as lifted higher? The front/left edge is higher, as well, (which might help with slide stop lever tab engagement when the mag runs empty).

This picture shows the oldest on the bottom and the newest on the top. Notice how the follower's legs/skirts have been changed, providing better lift and stability (under recoil).

If it were mine, I'd replace the mag springs and see if normal function was restored. If the .40 followers aren't the current ones, the company would probably mail you new ones for the asking.

Just my thoughts.

May 31, 2012, 09:31 AM
I suppose I should have specified that I was only 500 rounds into owning the pistol. I had bought it and was going through the training within a few weeks of purchase. I am sorry I forgot to put that part in there.Dont know if it matters.

May 31, 2012, 11:27 AM
It matters. If your gun is fairly new, the springs ought to be fine. (The 5K rounds-fired/5yrs-of-use recommendation for replacement.)

However, depending on how long ago it may have been produced (before being sold), the followers may, or may not, be the most current ones. You can tell if there's a "40" on top of them.

How experienced are you with shooting plastic-framed guns (or Sig's) that have the slide stop levers located above the shooter's thumb?

It's not uncommon for a shooter's thumb to sometimes interfere with the freedom of movement of a slide stop lever due to grip technique. Especially during recoil.

This usually happens when actually shooting the gun, but I've seen some people exhibit problems even when manually running the slide on an empty mag (by hand). Just depends whether they position their thumb on top (or beside) the slide stop lever.

If you actually detail-stripped the frame, hopefully you made sure that the slide stop lever spring was captured by the raised spots under the rear of the locking block. The picture below shows a couple of variations (.40 & .45 locking blocks) in the small raised "nubs" underneath the left side of the rear of the block, where the end of the lever's spring rests between them.

Also, when you removed the mag springs, were either of them bent or tweaked?

One of the standard armorer bench checks is for the magazine's follower to lock-back the slide (on each EMPTY mag, in an EMPTY gun) when the slide is briskly retracted and released (by hand).

May 31, 2012, 10:37 PM
Thanks for the pics guys. Whenever I have a moment during the day that I happen to think about this, I'll be following up with S&W. Been impossible to keep up at work lately.