Ok, so I picked one up yesterday.
I'm not sure how the thumb safety is going to work. The guns I typically carry have a large and relatively stiff safety. You can't miss them, and they're in the right place. The one on the shield is REALLY easy to miss. That is a software problem, but one that will take some conscious effort.
My initial thoughts on the gun are as follows: Fuck that. It is like shooting a Desert Eagle, but with nothing to hold on to. Ok, it isn't that bad - but - when the smallest gun you own is either a USP Compact or a P30, your 9MM M&P w/X300U probably has twice as many rounds through it than all of the other handguns you own combined, and the last time you fired a handgun was probably 18 months ago... the little fucker jumped a lot. I mean we're shooting a real caliber through a real small gun, it ain't designed to be fun to shoot.
To add to my problem, I had to used a modified Pat method of applying lubricant. In the absence of something to cut the tip off the new bottle of Slip2k, I unscrewed the top and poured it into the gun. This works just fine on an AR, but it might be a bit much for a pistol. If you've ever seen a chick with a nice (not fat) round ass in tight pants that could be described as "Two dogs in a gunny sack fighting over a greasy ham." - well, my shield was essentially a greasy ham. I ended up having to use an empty mag to lock the slide back when the line went cold.
This is not meant to be "Primary" CCW, this is meant to be a BUG and absolutely nothing else. I typically pocket carry a USP compact, but I can fit a P30L w/X300U in there without issue. This is what happens when you live in a city full of socialists and have to deal with people who will call the Police in a heart beat if they even think you're carrying a gun, but welcome violent fuckheads with open arms. I don't care that I don't enjoy shooting it, I only care that it reliably goes bang and can hit what I'm aiming at. To end on a positive note, I was very pleased with the accuracy.
Now then, with regards to the the title of this thread... I shot 75 rounds through each magazine. I got (I assume the standard) 1x 7rd, 1x 8rd. Sometimes they would take full capacity, sometimes they absolutely wouldn't. If the cap -1 round felt like the full cap round should feel, there was no chance on earth another one was going in. Speer Lawman vs Speer GD didn't seem to make a difference, and I'd say the issue started at about 50% and got worse until I said fuck it. I was there to get acquainted with the gun, and I could fuck with the magazines on my own time.
Having dealt with the H&K USP series 10 round mag clusterfuckery (ALL of my USP Fullsize .45 ban cap mags were absolute failures), I've got a pretty good idea of what is going on.
On the USP mags the spring would hang up on part of the absolute shit ban cap floor plate retention system. The floor plates were designed to absolutely limit the mag to 10rds, and the springs would hang on part of the assembly. Once the spring got caught, coils would overlap, the spring would be compressed past well beyond its spec, and the magazine was fucking worthless.
When I finally gave up, I observed the magazine and noticed that while the first round was right up against the feed ramp, the second round was not parallel. Viewing through the holes in the side, it looked like this was going on with each round relative to the previous. The bottom round was damn near parallel to the bore. At this point, I unloaded and re-loaded 5x to see if I could get a full mag, but it refused to cooperate.
It is likely the mag design is fine. I see you masochists up there with your multi thousand round counts and hundreds of magazines. The fact that this dudes magazines were black rather than the stainless everyone has mentioned... uh, ProMag? However, I'm having issues with standard factory mags.
It could be user error. The rounds are at one hell of an angle at the feed lips, and there isn't much room for a fat thumb to push down on the back of the top round while using the next round to push down on the front. I could be tilting the follower and causing all sorts of fun shit.
Bottom line, I think there is absolutely something happening to the spring - but it is entirely possible it is my fault. It certainly feels like the magazine spring is not tucking up under the follower as designed. I've got some shit going on this weekend, but I'll have a heart to heart with it next week and see if I can figure out what is going on.