So with my P938 and the extended magazine, I had an issue of where the magazine would unlock and cause a FTF.   This seemed somewhat common with other users and also the 380 version of this gun.  Since it only happened with the extended mag with its pinkie grip, I believe it was due to pressure applied to the pinkie grip stacking up with other issues.   And it seemed to occur most frequently with only a couple rounds in gun were the follower was in the magazine release slot perhaps exerting pressure outward on the release.

Anyway, I did relieve a small mold line on the inside of the magazine release, this is supposedly the Sig factory fix if you send it back in for repair.   This mold line can push the magazine outward away from the release ledge.  This helped a little but not much.

I also noticed it just took a small fraction of button movement before the magazine would be released and the spring pressure against pressing the release was on the low end compared to other pistols I own.

So, being it has a lot in common with a 1911, I took a 1911 Magazine Catch Spring I had as a spare.  Brownells part number is 965-011-152, however I think any fresh 1911 catch spring will do.   I had to remove two coils to get it to fit.  This made a huge difference and made the operation of the magazine release a much more deliberate affair.   However, it can still be easily operated when intended.

This also fixed the issue of the magazine unlocking during firing.

I still think the pinkie grip on the extended mag is not very well designed, I don't have overly large hands, but to get my pinkie on the rest requires me to get a very tight, overly firm grip on the pistol.   If the extension was a little shorter, it would be more comfortable to curl my pinkie around the bottom of the grip and still give a little extra support.   Ideally the grip would be a little longer and at the same angle as the front part off the grip and not curl forward.    I think its this curl foward that results in the pressure being applied downward promoting the unlock of the mag during firing.

Even though I still don't have the original issue, just to be on the safe side as I wish to replace my Colt Mustang as my CCW piece, I may take a hacksaw and my belt sander to the extension to reshape it.  Is it possible to remove the extension as it would be easier to modify and then re-install it.   It seems on there permanently.

Interesting, do you have a photo of the relieved mold line?

if you are member of sig forum, can see them here.  the line wasn't very pronounced, the extra strength spring made 10x the difference.

http://sigtalk.com/sig-sauer-pistols/29526-p938-7-round-mag-dropping-out-while-firing.html

http://sigtalk.com/sig-sauer-pistols/20715-p938-accidental-magazine-release.html

never had this problem at all.
maybe your grip or user error type?

I have had issues a couple of times with the extended magazine coming out.  After playing with it enough times, I figured out the problem was more me than the gun. The regular magazine slid in and locked fine, especially if empty. A loaded extended round magazine needed a stronger push to get it to lock in correctly. Me adjusting to a stronger push inserting the magazine took care of the problem. I almost think the plastic foot on the magazine combined with a full extended magazine and the known heavy springs in the magazine all contribute to the problem if you consider it a real problem.

Thanks for posting this. I am considering getting a P938 and would not want to buy a gun that it problematic.

sorry, not my first rodeo.   I have extensive experience with 1911s, Colt Mustang and even the Star BM which is essentially the same design as the P938.   I could duplicate this with about 80% consistency when the follower was opposite the mag release.   I also commonly practice one handed and some shooters may be keeping the magazine seated with their support hand.

Yes, I do have a death grip when shooting, but never had this issue with any other gun that uses a pinkie extension.  The particular combination of my hand size and grip strength along with the shape/size of the extension makes this issue bad enough to eject the magazine.  However, the magazine release effort with the factory spring is definitely on the very low end compared to other pistols I own.   The travel is also very short which makes it worse.

Improperly seated magazine may have been an issue one time when shooting the flush magazine.   Otherwise this issue was only encountered with the mag that had an extension.

FYI, another report of same issue:  http://www.handgunforum.net/handgun-forum-introductions/38086-new-p938.html

I would suggest replacing the mag catch spring.  The California complaint 238s use the same spring as the standard 938 and 238 models except one coil is cut off.  If a California spring inadvertently got mixed in the parts during assembly, it could cause what you're finding.

No problems like this out of mine.

you will NEVER find any gun that has no problem at all. Most P938s work just fine.

Only time my 238 or my wifes 238 drops the mag is when the release is pushed, with both the 6 and 7 round mags...