There are manufacturing differences between magazine catches. The FS and compact use the same catch, however tolerances are not held very close.

I ordered a replacement catch from SSS last week for my compact. The blade on the replacement (marked 2) was visibly shorter than the original (marked 6). Measurements confirmed that the #2 catch was .015" shorter than the #6.

I installed the catch in the reversed position and put 100 rounds through it Sunday with no issues. Three different magazines were used with unmodified followers.

I compared the catches on both my FS pistols. The 9FS had a #2, the 40FS has a #1 marked catch. Both of these catches measure within .005" of the replacement catch I received from SSS.

I'll run another couple of boxes through the compact this weekend and see if the new catch continues to work properly.

Just an FYI for people having this problem:

I sent my FS 9mm and magazines to S&W last week for this (and another) problem, and got it back today. One of the repairs is listed as "ADJUST MAGAZINE." The adjustment appears to have consisted of just what ottawadriver suggests; they reshaped the followers in all 4 magazines, so they don't catch, by filing down the corners.

Thanks for the update. I won't waste time sending mine in. Dremmel fix is working great over 500 trouble free rounds on 3 different guns. Interesting I got a 40 c last week and not one issue with no adjustments. Different follower though

Well looks like the answer has arrived. In my emails back and forth with Customer Service, I sent a final one with all my findings, the pics, and exactly how to fix it. I asked that they forward the email to the engineers and repair techs. They replied that they did forward it to them. I think they should send some more mags or some other swag........

Installed two new followers from MidwayUSA. I took the slide off, took the floorplate off of the mags, and took the front corner off little by little. Made sure it easily went by the mag catch, and still it the slide lock. It doesnt contact the slide lock square, but has plenty of surface to hit the lock. Tested yesterday... works flawlessly!

Almost three months later and no problems with the pistol.

I am noticing another issue that I need to work out. The slide release for left handed operation is offset (farther forward) than the opposite side, and it seems to be more difficult to actuate. Anyone have any suggestion for smoothing it out and allowing easier operation? Maybe a slight polish to the contact point on the slide and catch?

My wife's full size M&P 9 had the same problem (she is a lefty). S&W sent me 5 new followers and still had same bind of follower at left side mag catch cutout. Called S&W again, guy reviewed invoice, noted that they had sent the newest revision of follower and said they needed to send the previous revision of the mag follower, which they did, and we installed and tested extensively, experiencing zero failures. Problem solved, look for the old style follower!

I just pruchased an M&P9 full size last month. Worked great for the first week. During my first cleaning I swapped the release over for left handed shooting. After that it was like ever 7th or 8th round would jam in every magazine. Did a search and hit this thread of great information. Test fitted an empty mag and popped the follower down with a pen and sure enough it snagged on the release poking thru the hole on the left side. I was getting ready to pull the mags apart and modify them as shown here this weekend. This morning I thought I should call S&W and glad I did. Apparently they now have a fix or a replacement mag release switch for left handed installation that doesn't interfer with the follower. Apparently it doesn't matter about the caliber but does if it's full or compact size. Should arrive in 5 days and I'll let you know how it goes.

I think it's time to give ol' Smith a call! I just got a 9c last month. I only ran one mag through with the catch reversed. It did it an I switched it back for now. Now if they can only solve the problem with the paint coming off of the front sight post. The FS made it 1000 rounds, the 9c made it 25.

Thanks for the update!

Quick update. I never received the new mag release so I called again this morning. The person I originally spoke with is no longer there. Today I was told they are aware of the problem and their fix is now to replace the followers with what he called "old style". So they are sending me a couple of replacement mags with the "old style" followers.

I just bought a 9 fs and switched to southpaw mag release. Upon the very first 50 rounds I encountered at least 6 ftf, 4 stove pipe ejections and numerous failures to lock the slide back on empty mags. Read through the info here and switched back the mag released for rights. Fired 100 rounds and no problem. Iam about to send it back to SW. My concern is that if they correct it for southpaws would this eliminate my possiblilities to switch it back to the standard fleet side mag release. I liked the feature to make the change as I groom for familiar with my shooting preferences. I would hate to lose it. Thanks for all the info guys.

First I wanna say thanks to all of you who described this problem and the fix. I'm a lefty and recently started having this problem. I brought home some sand paper from my toolbox and fixed the problem in 2min. Glad I don't have to send off the gun to s&w any more.

P.s. This is my first post. Thank you guys for the awesome information.