Bought my .40 shield about 2 weeks ago and i am having the same issue with the mag. dropping out particularly the extended one
I havent removed anything from the gun The magazine springs are so tight it is not humanly possible to load by hand without some sort of device to help, the springs in the magazines are really stiff and tight. Im thinking if u have the same issue, it could be the magazine spring is so tight that when the round releases when you shoot the gun, there is so much pressure on the spring that it is affecting the catch on the inside of the gun, from the percussion of the shot.
I did notice that if you dont fill the magazine entirely, it doesnt happen
After the spring loosens up im hoping it resolves the issue, if not, the service dept will be getting a call

As a follow up, load the magazine fully and let it sit, too weaken the spring, so there isnt so much pressure on the spring

Problem solved. Wasn't sure if it was my grip, or a faulty mag release. Ordered a Costa Ludus Extended mag release. Fit perfect, and works great. 300 rounds without a mag dropping. Got my confidence in the gun back. I'm going to order a stock release from S&W, but I like the extended release now. I can catch the release quickly without having to break my grip. Works good with gloves too..
Just an FYI..

I bought my .40fs in 2006 and never had this issue.

Originally Posted by Bill340

Bought my .40 shield about 2 weeks ago and i am having the same issue with the mag. dropping out particularly the extended one
I havent removed anything from the gun The magazine springs are so tight it is not humanly possible to load by hand without some sort of device to help, the springs in the magazines are really stiff and tight. Im thinking if u have the same issue, it could be the magazine spring is so tight that when the round releases when you shoot the gun, there is so much pressure on the spring that it is affecting the catch on the inside of the gun, from the percussion of the shot.
I did notice that if you dont fill the magazine entirely, it doesnt happen
After the spring loosens up im hoping it resolves the issue, if not, the service dept will be getting a call

The 40 shield mag drop issue is real. Call S&W for the fix. They will take care of it.

Originally Posted by Bill340

As a follow up, load the magazine fully and let it sit, too weaken the spring, so there isnt so much pressure on the spring

This won't work.
Springs "weaken" from repeated flexure, not from just sitting around in a compressed state.
Compressing a new spring will shorten it a very little bit, but any magazine's spring is always longer than the magazine, so there will be no measurable effect upon its strength.

This has been discussed here before. Use the "search" tool and see.