6906 Slide Lock Back Issue

[indymike]

When I take it to the range, it doesn't lock back about 1 out of every 2 mags. After the last round, it slams forward and leaves the hammer cocked so I know it's doing a full cycle.

If I insert an empty mag and slowly pull back the slide, the slide stop lever snaps right into the slide catch cutout on the slide with no problem. I've done this a million times fast and slow so I'm really at a loss at what is going on during shooting that keeps the slide stop from grabbing the slide in time. The notch isn't worn at all. From the looks of it and the looks of the barrel rifling, I don't think this gun saw a lot of range time.

I've tried 115gr, 124gr standard and +P ammo and 147gr ammo. It does this with all the ammo. I have a 3914 that I take with me using the ammo and no slide lock issues there so I pretty certain it's not the ammo.

Things I have tried:

-New mags from Midway with double slotted mag catch holes
-Preban mags with one mag catch hole, new mag spring AND old ones.
-Ban era mags with the LE markings, new springs and old springs.

A few other things to note. This gun has a new recoil spring I got from Brownells. It's S&W standard strength, not Wolff. The mag springs are S&W standard strength as well.

Also, the only thing other than the recoil and mag springs I've changed on the gun is I put a new magazine catch nut and mag catch nut spring in the gun since the mag release was pretty mushy. Is it possible that the mag catch nut can be too firm or loose to cause such a problem? Or is it possible that the mag catch itself is the problem? Lastly, is it possible the mag and recoil springs are out of sync? It's as if the recoil spring is too strong and returns the slide forward before the mag spring/follower can catch up. Have no idea, just throwing ideas out there.

Any thoughts from experienced 3rd gen owners would be greatly appreciated. My next steps are to try getting a new slide stop lever and mag catch lever but they are out of stock everywhere.

 

[Arik]

......

Sent from my SCH-I545 using Tapatalk

 

[Fastbolt]

Does it happen when you use new mag springs and shoot the gun left-handed?

 

[Gunhacker]

It sounds like you've tried different magazines and had the same issue... what you might start to suspect is the slide lock/release lever itself.

The first thing I'd check is see if there is any binding in the spring loaded plunger on the end of the lever that pushes end of the side plate "arm".

It also not unheard of for the end of the side plate the plunger engages, to get bent, (especially while removing grips if you don't take care) and place more resistance on the slide lock/release lever than normal.

The other thing that places some tension on the lever is the spring loaded plunger on the end of the recoil spring guide rod, if that plunger is binding, it also could add enough resistance where the lever doesn't freely rotate upwards under the spring tension from the magazine follower.

For grins and giggles, do you have another 3rd gen that you can swap the lever with as an experiment?

Those are my best guess...

 

[Fastbolt]

Usually, when a slide fails to remain locked back on an empty mag, it's probably:

Damaged mag follower
Damaged or worn mag spring
Dirty mag
Damaged slide stop
Damaged slide plate

Those are the mechanical "probable causes". Don't discount potential shooter-caused issues, though, which is why I asked about whether the gun did the same thing when shooting it left-handed.

I've watched a couple other armorers drive themselves in circles trying to figure out why a couple right-handed 4006TSW shooters were experiencing early slide lock-backs, or failures-to-lock back, trying to fix the guns. The problems disappeared when I had the guys shoot the guns left-handed, however, and careful observation revealed the shooters were unintentionally pressing against the slide stop levers during recoil.

Another guy brought me a P239 because the owner couldn't get the slide to lock back on an empty mag ... but the problem disappeared when the gun was fired left-handed. In this case, the owner was used to firing a P229 and keeping his thumbs out of the way of the slide stop lever, but the slightly smaller gun caused him to position his thumb exactly where it interfered with the slide stop lever during recoil.

Now, if you've eliminated the follower & mag spring issues by using new S&W factory mags, springs & followers ...

... and the new recoil spring is the factory spring (painted green, and the same spring used in the 3913) ...

... then you might look at the slide stop lever and side plate.

The slide stop lever ought to have a roughly even space between the back and the outside of the frame, for the length of the lever, and the lever's inside tab ought to easily reach the follower's shelf. If the gap between the lever and the frame is a lot wider at the rear, the lever's tab might not be catching the follower all the time. The plunger ought to be seated against the angled plate of the side plate, not protruding to the outside or caught below the bottom edge of the plate.

The side plate ought to be fixed, meaning you can't wiggle the angled part of it (which touches the slide stop lever plunger) up & down with gentle finger tip/thumb pressure. If the legs (or prongs) which are captured by the left end of the headed sear pin are bent too far in the wrong direction, it could increased the angle of the side plate at the top/front (where the lever plunger rests against it), and maybe prevent the lever from rising consistently high enough under the power of the mag spring lifting the follower.

The mag catch isn't usually involved with this sort of failure-to-lock back problem. BTW, the mag catch button's plunger ought to be flush, or slightly below, the frame's surface. (It's adjusted by turning the mag catch button 1/2 turn at a time, and the plunger snaps up into one of the 2 recessed slots on the bottom of the button, located 180 degrees apart from each other.) Any roughness or crunchiness of a new spring, plunger & button can usually be solved by a judicious whack of the mag catch button (assembled in the frame) with a plastic mallet, smoothing any rough edges. Roughness in older parts can often be caused by corrosion/dirt inside the mag catch button parts & frame hole.

 

[indymike]

I have pretty much eliminated these three by using 6 different mags, new and old, new springs and old springs.

-Damaged mag follower
-Damaged or worn mag spring
-Dirty mag

I was also shooting a 5904 at the range and didn't have this issue so I'm guessing my thumb isn't hitting the slide stop lever. I'm not ruling it out, I'll definitely pay attention to it next time I go to the range but I can honestly say it's unlikely since I've been shooting 3rd gens since 2004 without this problem ever popping up before.

Regarding the slide stop lever and the side plate assembly, I think there might possibly be an issue there. I've posted pictures below comparing the 6906 to a 5904 that works just fine. The position of the slide stop lever against the side plate assembly seems ok.

However, the slide stop lever seems to stick out from the frame a bit more than the 5904. Also, you can see that the slide stop leg or arm, whatever you call the part that traverses the frame, it doesn't quite stick out of the frame as much as it does on the 5904. I've tried to push on it really hard to get it to go in more but that's as far as it goes. Is that normal?

And for the side plate assembly, I did pull it out a bit so that I could install a spurred hammer on this 6906 and when I put it back in, I noticed the top pin/leg didn't seem to go all the way in. I don't know if it was like this before I installed the hammer. But again, I pushed it in as far as I could and on the side with the side plan assembly, it's flush with the frame. It's not on the other side though, maybe it's bent?

Thanks for all the suggestions and help, I really really appreciate it!

 

[Fastbolt]

To most accurately diagnose a problem, it helps to limit the number of influences and factors involved.

Do you have a good 6906 mag & follower with a new factory 69XX mag spring?

Then, I'd be curious if the same problem occurred if you used the 5904 slide stop lever assembly in the 6906.

If the problem doesn't occur with the other slide stop lever assembly, then that's a hint. FWIW, slide stop lever assemblies aren't exactly greatly available at the moment, so finding a new one could be a little difficult.

Don't discount the possibility of the slightly different grip shape (shorter) and subtle felt recoil difference having an unrecognized influence on your grip when it comes to thumb placement & tension under recoil, especially if you use a crossed-thumbs grip. hence, the question about shooting it left-handed ... unless you ARE left-handed, of course. Are you?

Can't tell anything about the mag catch without holding the gun and checking function (holding & releasing the mag with normal pressure & movement). The recommendation for the top of the plunger to be either lower or flush with the outside of the frame is the standard one, but tolerances can vary among guns and parts. I've come across an occasional gun where the plunger ended up looking a bit high, but the mag catch worked normally and did what it was supposed to do.

Removing the sideplate can risk damage to the plate's legs (prongs), and somewhere around this forum I posted another set of pics showing how install the sideplate's legs over the sear pin, and what a normal installation ought to look like when done. If the legs are bent or broken, they can't hold the sideplate properly stable at the front.

If the original 6906 slide stop lever is damaged (tweaked/bent outward too far at the back), it might be possible to adjust it, but the "correction", if done improperly, can just as easily really damage the elver assembly. They no longer teach armorers how to "adjust" bent levers, but tell them to just replace them. (It involves the use of a vise and a lead babbitt, and some carefully applied whacking. )

Let me know if exchanging the slide stop levers changes anything in this failure-to-lock-back issue in the 6906.

 

[indymike]

I'm not left handed so the thumbs are still a possibility. I was shooting my M&P Shield in the same range session so the more I think about it, the more I think I may have had an altered grip with my thumbs riding where they normally don't.

I'm really not sure if I have a good mag, follower and spring since I only have one 6906 and the fail to lock back problem has occurred with them all.

In the picture from L to R:
1. used, old stock with used spring
2. new, old stock with new spring
3. new mag from midway I purchased in Jan (I have 4 of these)
4 and 5. Used LE marked mags with new S&W springs from Brownells
6. new, preban mag with new, old stock spring
7. used, preban mag with new S&W spring

I checked the followers on all of the mags and even the used ones look new.

So for the next range trip, I'm going to take all the mags and the 5904 slide stop lever. I'll first try each mag using my left hand and/or watching very closely where my thumbs are. If I still have the problem, I'll swap out the slide stop lever and go from there.

Thanks a million for taking the time to help with this. I really appreciate your sharing of your experience.

 

[Fastbolt]

De nada. You're welcome, if it helps.

Don't feel too bad if it turns out to be an errant grip change, though. I've seen some longtime shooters experience a subtle and unrecognized thumb movement when changing to a new gun, even if it's just a different sized model of an existing pistol with which they're familiar.

I've seen a couple guys who use high-thumb & crossed-thumbs manage to shift one or the other of their thumbs - under recoil, when they wouldn't necessarily notice it - so they either pressed against or on top of the slide stop lever.

In some other instances different shooters have pressed slightly upward on the lever, which can quickly be annoying when it causes early lock-back, with ammo remaining in the mags.

If it's not the thumb(s), but turns out to be that slide stop lever that has the lever slightly bent outward (meaning the angle between the lever and the lever assembly's pin exceeds 90 degrees), a skilled gunsmith or armorer can still sometimes salvage the lever by adjusting the lever to restore the proper angle.

If it's something that's become damaged (or corroded) inside the plunger's hole, or just some grit that's now preventing the plunger from moving normally (compressing inside the end of the lever easily enough to let the lever rise upward against the decreasing angle of the sideplate's little angled plate), it can be disassembled and cleaned out, or the spring and/or plunger replaced.

Removing the itty bitty roll pin and replacing it, though, can still create a potential situation where the roll pin isn't correctly aligned with the plunger & spring during installation, perhaps risking damaging the plunge as the roll pin is driven in. (In a similar manner that incorrectly positioning the firing pin in a classic Sig can risk having a burred edge raised on the clearance cut when the retaining pin is hammered back in the slide.) I suspect that's one reason 3rd gen armorers are no longer taught to disassemble and replace the small roll pin, plunger and spring in slide stop assemblies.

Hope your problem is easily identified and corrected. My issued 6906's were some of my favorite duty weapons over the years. I tried to wear one out by firing an estimating 45K+ number of rounds through one particular frame some years ago. Yes, there were some normal maintenance parts/springs replaced during that time, as well as a chipped extractor (at about 12K rounds), and I always made sure to keep the rails properly lubed to help prevent excessive wear to the alloy frame rails. I also went through some recoil springs.

FWIW, the standard recommendation to 3rd gen armorers is to replace the recoil & mag springs either every 5K rounds fird or every 5 years (since LE guns are typically left with constantly loaded mags). I tended to replace mine a bit sooner, meaning every 3K rounds (give or take), as I suspected the shorter & faster cycling 6906 slide probably resulted in faster wear to the recoil springs. (I was also using a mix of standard pressure 147gr loads, as well as some 127gr +P+ and 124gr +P duty ammo during that time, and the hotter loads can accelerate general expected wear & tear, as well as decrease recoil spring life.)

Oh yeah, one other thing I forgot to mention is to make sure the mag springs are installed in the correct orientation, so the front of the follower has the necessary tension. Otherwise, either feeding stoppages could occur, or the front of the follower might not be lifted high enough, consistently enough, to catch and lift the slide stop lever on an empty mag (when the mag spring is at its least amount of tension). Just another thought. I've seen some upside down and/or backwards springs in mags ... and I've even seen some mags where the issued users put the followers back in the mags backwards. Weird, but it can happen.