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Discussion Starter #1
Got a new 9mm shield. Took it to the range last weekend. If using a full mag, 7 or 8 round, the slide would not lock back after the last shot. Maybe did it 1 or 2 times out of 20 mags. Load just one round and it would lock back less than 50% of the time. Tried this 6 or 8 different times over 30-45 minutes. In each scenario if I pull the slide back manually after last round the slide locks back properly. Put an empty mag in, pull slide back it locks back properly. Gun was cleaned and lubed properly before range trip. Was using Winchester white box 115 gr brass ammo. Same ammo was used in USPSA match 30 minutes prior in M&P FS 9mm with no issues so I don't think it is ammo related. Anyone else experience issues like this. Do I need to send it back to S&W.
 

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OK, I'm going to assume it's not a magazine related issue.

Sorry, but I need to ask... your thumb isn't doing it is it? Or it's not being limp wristed?
 

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Are you lubing the recoil spring assembly...??? I`ve seen some pretty tight shield RSA`s cause this issue. Spray the hell out of the RSA with dry lube, let it sit for 5 minutes while the dry lube dries. Then see if the slide locks open on your last round fired. If it does not, call S&W and have them send you a new RSA.
 

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Discussion Starter #4
RSA was sprayed with drylube as you describe. Thanks to this forum I had read that piece of advice before the range trip.

Limp wrist I seriously doubt. Since this is a much smaller gun than the FS I am used to, the thumb is something I should double check.

Thank for your comments
 

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Sorry, but I had to ask the obvious. It's like when something electrical doesn't work I always remind to check to make sure it's still plugged in.
 

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Often this is ammo related, I got a Pro Series 9L a couple of years ago, I was having a lot of jams, failure to eject. This was my standard 9mm load I had used for 30 years and had always worked perfectly.

In this case I needed to change the recoil spring, I went 1lb lighter and it completely cured my problem.

A load that is too light is also a possible cause of failure to lock back, the solution is to adjust the load or experiment with a slightly lighter recoil spring. A lighter recoil spring will allow the slide to go further to the rear allowing it to lock, don't go too light or you get battering on the frame, generally you reduce the rating 1lb at a time.
 

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Would also try locking slide back and leaving it for several days. Might be a very tight spring that needs break in.
 

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If your thumb is riding on the slide lock then it won't lock back on the last shot. I had this problem and that's why it happens just some of the time. Check it out.
 

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I would try some other ammo wwb is not know to be very good ammo, it most likely does not have enough power to push the slide all the way back
 

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I just saw this old discussion when searching for an answer to my problem. My M&P Shield 45 slide wouldn’t lock back, after shooting the last round in the magazine, yesterday at the range—never had that problem before. After some investigation, I discovered that the mag springs had just relaxed somewhat, due to my keeping them fully loaded with ammo all the time (and thus compressed), since this is one of my concealed-carry weapons. I took a mag apart and stretched the spring back out a little longer and reassembled— PROBLEM SOLVED! Now the slide locks back every time as designed. I guess I’ll do the same for all of my other mag’s and then shoot more to confirm success...hope that helps someone!
 

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I just saw this old discussion when searching for an answer to my problem. My M&P Shield 45 slide wouldn’t lock back, after shooting the last round in the magazine, yesterday at the range—never had that problem before. After some investigation, I discovered that the mag springs had just relaxed somewhat, due to my keeping them fully loaded with ammo all the time (and thus compressed), since this is one of my concealed-carry weapons. I took a mag apart and stretched the spring back out a little longer and reassembled— PROBLEM SOLVED! Now the slide locks back every time as designed. I guess I’ll do the same for all of my other mag’s and then shoot more to confirm success...hope that helps someone!
Stretching any spring “back out” as a fix for any spring related problem isn’t a permanent solution.

Absent another problem; you need new mag springs. That said; I have mags (M&P including Shield) that have been used, then fully loaded, for years with no problems. Springs don’t weaken by being left either loaded (compressed) or unloaded (extended). They weaken over time by being cycled (compressed/extended).
 
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