I have 3 magazines for my model 5903. Two of the magazines (which are marked S&W) fail to lock the slide back following the final shot. They also fail to lock the slide back when opening the slide (with no cartridges in the weapon). I have tried replacing the springs and followers, but this has not solved the problem. Is there something else I can do? Or do I just replace the magazines with new ones (such as Mec Gar)? Thanks for helping me.

When you say you replaced the followers and springs, did you mean with brand new ones, or just from other used magazines?

While a failure for the slide to be locked back on an empty mag is often a sign of weakened mag springs or damaged/worn followers ... (or a very dirty mag body) ... it can also be indicative of a problem with either the slide stop lever assembly or the side plate.

FWIW, I've seen a couple of people install mag followers backwards, which puts the follower's locking shelf (which engages the slide top lever tab) on the wrong side of the assembled mag body. The last time someone brought me a gun where they'd reversed the followers, the gun was still feeding normally during a qual course-of-fire, but the slide wasn't locking back when the gun ran empty. Weird, but not the weirdest thing I've ever seen someone do.

Does the slide stop lever plunger move freely (when removed from the frame during field-stripping)?

Does the plunger look like it's being trapped either to the outside or underneath the side plate (small angled plate the plunger rests against)?

Does the little angled plate of the side plate move up and down with gentle finger tip pressure, or is it solidly in position? (It's not uncommon for someone to unintentionally damage a side plate when removing and reinstalling the factory grips, usually by tweaking the side plate legs by exerting excessive force against the top of the side plate.)

No way to know what's happening with your 5903 without being able to inspect it, but those are some common issues to check. Just some thoughts.

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Ret LE Firearms inst & armorer

Thank you for your thorough and thoughtful reply. In response to your questions:
1) The mag springs and followers were replaced with new ones.
2) The mags are clean inside (not lubricated) and out.
3) The mag followers are inserted correctly (i.e., they are not in backwards).
4) The slide stop plunger moves freely.
5) The plunger is not trapped either to the side or underneath the side plate.
6) The side plate moves slightly with pressure as you described. The grips have never been removed.

One of the 3 S&W mags works perfectly...but the other 2 fail to lock the slide back. All 3 mags load and feed normally. Thanks for your excellent suggestions. I have learned from them.

De nada.

Wish I could examine and test-fire the gun, as well as watch you shoot it. You right-handed? If you're left-handed, you can ignore some of the following ...

The reason I ask is that I've come across a few right-handed shooters who were experiencing either premature lock-back (with rounds remaining in the mag), or failure of the slide to lock-back on an empty mag.

The last couple of times it was puzzling to the other instructors working with the shooter, so they called me over. Once a brief examination of the gun revealed no obvious problems, I had the shooters fire rounds while I watched, making them focus on making rapid, accurate shots (to put them under a bit of minor stress).

While their support hand thumbs weren't near the slide stop levers before they fired each round, or after each shot was done, the rising/falling of the cycling gun caused them to shift their support hand grips just enough to let them bump, or press, against the slide stop lever during recoil. They didn't believe me, so I ahd them shoot the guns left-handed (putting their thumbs on the opposite side from the slide stop levers).

When the "problem" went away while they were shooting left-handed, they acknowledged a possible need to revise their grip technique (thumb placement) a little bit.

I've also had this problem occur with an occasional Sig shooter, especially when going from a full-size to a compact model with a right-handed shooter.

So, if you're right-handed, and you're able and comfortable shooting left-handed, you might try it and see if the problem still occurs. If so, we're back to either a gun or mag problem. If not?

Also, if the "problem" consistently follows a couple fo specific mags, I'd simply try to obtain some other mags (either factory or Mec-Gar) and see if the problem is resolved. Sometimes a mag body wear, damage or spec problem may not be visible to the naked (and even trained) eye, but can still cause problems, nonetheless.

Lastly, don't discount the potential for an occasional ammunition influence to rear its unexpected head. I usually default to test-firing with one or another of the major American brands of quality 9mmP ammo. Duty/defensive type, if possible, but budget brands if necessary. It's not all that surprising if a company's "budget" line of ammo might not use all the same quality components, is it?

Even variations in power (slide velocity) c an have an adverse effect on side travel and cycling speed, and might cause some erratic slide stop lever/slide notch engagement issues. Just depends. Couple either lower or over-powered ammo with an unfortunate lack of a solid grip (firm - not death - grip, and a locked wrist), and sometimes weird issues can pop up and make someone scratch their head.

One last thought. How old/worn is the recoil spring? Too fresh ( at peak of tension) and you can get some resistance to slide travel and run, especially if couple with a hesitant grip or lower powered ammo. Too weak/worn, and the slide might be traveling just too fast "enough" to bounce away from the follower being able to lift and capture the slide.

Obviously, this sort of thing can be difficult, if not impossible, to diagnose and resolve in an online manner. It really requires being present to examine the gun and observe the shooter (using the particular ammo, mags, etc).

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Ret LE Firearms inst & armorer

I thought about the possibility of the thumb contacting the slide stop lever, but that is not the case. One mag functions perfectly every time and the other two do not. Also, thumb in contact with the slide stop would not explain failure to lock back when opening the slide with an empty mag inserted.
Regarding ammunition: I am using the same ammo in all three mags. Don't think that is the answer, but it is a pertinent question.
About the recoil spring: I sent this pistol to S&W for their inspection, cleaning, and replacement of springs a couple of years ago. And it has been shot very little since then.
I came to the same conclusion as your about ordering some new Mec Gar magazines to see if that solves the problem. They will be here in a few days, and we will have some additional data to help us.
Thank you for taking your time and effort to help me. You have gone "above and beyond the call of duty," and I'm truly appreciative.

@Fastbolt: As a followup to our discussion, the new Mec Gar magazines work perfectly in the 5903. The followers in the new Mec Gars have a slightly different profile from the ones I had been using. Thus it is my impression that the fault lay with the older followers, as you had suggested. It is nice to have this problem solved. Thanks again for helping me.

De nada. Glad it turned out to be a simple fix.

As with many other things in life, "fixing" a gun problem is a lot easier if you approach it from the perspective of trying to diagnose the actual problem(s), instead of trial & error. Sometimes a "problem" is actually just the "symptom" of a problem that's different than expected.

BTW, if the followers were the older black ones, they might just have become worn in the wrong manner. FWIW, the orange ones came in at least 2 distinct variations with which I'm familiar (which doesn't rule out there may be others, of course).

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Ret LE Firearms inst & armorer

The followers were the orange ones...and based upon what you said, I'm doubting that got the correct ones when I replaced the springs and followers. Being a traditionalist, I really like the idea of having the original mags that came with the pistol. But the replacements seem to be a step-up in quality and reliability. Guess you can't have it all. Thanks for your reply.