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Causes For a Semi Auto Pistol to Not Lock Back On an Empty Mag?

RLinNH

Sergeant of the Hide
Full Member
Minuteman
Mar 23, 2019
333
194
I have had an FN509 Tactical for a little less than 2 years now. It has grown into my favorite striker fired pistol. It really has very little recoil and is more accurate than any other striker fired pistol that I own. I have 975ish rounds through this weapon. Just recently on my last range visit, the slide stopped locking back on an empty mag after firing. I am thinking that maybe it just needs to be cleaned (which I find hard to believe as I only have 230 rounds through it since it's last cleaning), so I'm going to break it down and scrub soon. I was also thinking that maybe I should change out the spring in the upper (comes with 2 springs). After reading the manual, I already have the "softer" yellow spring installed. I'm thinking that the "stiffer" silver spring would only cause the slide to not want to lock back on an empty magazine even more prevalent. These are the only 2 issues I can think of that would be causing the non lock back. Other than something is broken. Anywho, as mentioned above. I will scrub the pistol and not change the spring. If after I clean it and it still fails to lock back, I'll then change the spring. And if that fails to fix the issue, than I am going to figure that something has to be broken. Sound about right? And, did I miss anything in regards to causes to keep a slide from locking back on an empty Magazine?
 
I have had an FN509 Tactical for a little less than 2 years now. It has grown into my favorite striker fired pistol. It really has very little recoil and is more accurate than any other striker fired pistol that I own. I have 975ish rounds through this weapon. Just recently on my last range visit, the slide stopped locking back on an empty mag after firing. I am thinking that maybe it just needs to be cleaned (which I find hard to believe as I only have 230 rounds through it since it's last cleaning), so I'm going to break it down and scrub soon. I was also thinking that maybe I should change out the spring in the upper (comes with 2 springs). After reading the manual, I already have the "softer" yellow spring installed. I'm thinking that the "stiffer" silver spring would only cause the slide to not want to lock back on an empty magazine even more prevalent. These are the only 2 issues I can think of that would be causing the non lock back. Other than something is broken. Anywho, as mentioned above. I will scrub the pistol and not change the spring. If after I clean it and it still fails to lock back, I'll then change the spring. And if that fails to fix the issue, than I am going to figure that something has to be broken. Sound about right? And, did I miss anything in regards to causes to keep a slide from locking back on an empty Magazine?

A stronger recoil spring might or might not make things worse. That depends on what the real root cause is.

You missed quite a few possible reasons for your problem:
  • A portion of one of your hands is stopping the slide stop from moving up on the last shot
  • Magazine springs weak, not pushing up on the follower hard enough. The follower is what pushes the slide stop up to lock the slide on the last round
  • Lack of lubrication. Specifically on the slide rails on the frame and/or the grooves for said rails in the slide.
  • Using weak ass ammo that doesn't have enough recoil to send the slide back far enough
  • Any combination of the above reasons
 
Inadvertently resting your thumb on the slide release will cause this also.
 
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When was the last time you cleaned your mags?

Forgot about that one. Crud in there can prevent the follower from moving up that last little bit to push on the slide stop.
 
well you def shouldn't have to be changing any springs after only 1k rounds.

how many mags do you have and is it not locking back on all of them?

did you change ammo at your last range trip, maybe softer?

if you manually rack it all the way back, is it staying locked back?
 
Try this:

Take the slide off
Insert an empty mag
When the mag locks, the shelf on the mag’s follower should be making contact with the lower edge of the slide stop
The follower is what pushes the slide stop up which is what locks the slide back when the mag is empty

If it’s not, it is possible that your mag springs are weak. You can try stretching them out a little before re-assembling your mag or replace the springs.
 
When was the last time you cleaned your mags?
Never. Valid point. Also, I have recently changed my handgun grip to the thumbs resting along the slide of the pistol (was taught how to shoot handguns with a revolver. So this "new" grip wasn't a grip back then). The new grip I am using has SIGNIFICANTLY improved my handgun accuracy. I'll have to pay attention to where my thumbs are resting while shooting.
Thanks to all for posting as I am anxious to get to what is causing this. I have feeling it's Operator Head Space and Timing...:oops:
 
I have recently changed my handgun grip to the thumbs resting along the slide of the pistol (was taught how to shoot handguns with a revolver. So this "new" grip wasn't a grip back then). The new grip I am using has SIGNIFICANTLY improved my handgun accuracy. I'll have to pay attention to where my thumbs are resting while shooting.

While the slide stop on your pistol isn't huge, it's still possible that you might need to ever so slightly modify your grip to not hold it down.
1703436786229.png



Whatever you do, don't go back to this grip or anything like it
1703436840733.png
 
A stronger recoil spring might or might not make things worse. That depends on what the real root cause is.

You missed quite a few possible reasons for your problem:
  • A portion of one of your hands is stopping the slide stop from moving up on the last shot
  • Magazine springs weak, not pushing up on the follower hard enough. The follower is what pushes the slide stop up to lock the slide on the last round
  • Lack of lubrication. Specifically on the slide rails on the frame and/or the grooves for said rails in the slide.
  • Using weak ass ammo that doesn't have enough recoil to send the slide back far enough
  • Any combination of the above reasons
All of what @308pirate posted above are the first things I’d check.

Emphasis on the first and fourth bullet points.

Are these handloads?

Assuming the mags are only as old/used as the pistol, with less than 1,000 rounds through the weapon, failure to lock back is kind of weird unless it’s user or ammo-induced, but it’s always possible someone effed up the heat treat on a batch of mag springs or recoil springs. Also possible something is wrong with the slide stop mechanism itself, but not likely.

My guess would be you’re getting your thumb on the slide release without realizing it.
 
While the slide stop on your pistol isn't huge, it's still possible that you might need to ever so slightly modify your grip to not hold it down.
View attachment 8304701


Whatever you do, don't go back to this grip or anything like it
View attachment 8304702
HaHaHa!!! That looks a lot like my old handgun grip. It wasn't that bad, but it wasn't good either. I don't see myself going back to my old grip as the accuracy difference with this new grip is THAT Dramatic!!!
 
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HaHaHa!!! That looks a lot like my old handgun grip. It wasn't that bad, but it wasn't good either. I don't see myself going back to my old grip as the accuracy difference with this new grip is THAT Dramatic!!!
Shoot the pistol with just one round in the mag. Do it strong hand only with the thumb pointing up. If it locks back you know it's your hands interfering.