If you mention what you didn't understand someone might be able to explain it to you. There aren't any stupid questions here, except the ones I ask.

I've searched around the various forums for answers to this accuracy question and haven't yet come across a thread that explains it. If anyone knows, they aren't spreading the news with details. Another issue you will hear quoted often is something about lockup timing - again without specifics.

So I have come up with my own theory but of course cant prove it due to lack of data and a lot of the supporting comments are hearsay. It goes like this...

Lockup refers to when the barrel and slide are fully forward and ready to fire. The parts involved are the recoil spring for tension and the barrel hood extension that fits in a recess at the slide breech face. The barrel and slide should be held in place together fairly tightly.
How well the barrel and slide fit together, in the same repeatable alignment shot after shot, will have a lot in determining that guns accuracy or lack of it. For whatever reason (by design, manufacturing tolerances, different subcontractors etc) the barrel and slide can have slop from an accumulation of tolerances.
I think that the more play, slop, gap there is between the barrel and slide at the lockup recess, the less accurate the gun will shoot because the barrel will be pointing slightly different than the sights each shot. Some people have reported using a stiffer recoil spring has helped a little. YMMV

If you slowly pull your slide back you'll see the rear of the barrel slides downward and the muzzle tips upward. This is the process of unlocking. At some point the slide will continue rearward extracting the shell case, stripping another off the magazine and feeding it into the chamber and the barrel will slide back up into locked position, all thanks to the recoil spring.

Under the barrel, (below the chamber block) is a slot that fits in a bar in the frame, that cams the rear of the barrel down as the slide travels rearward. The shape of this slot determines the lock/unlock timing of the barrel cam action. You would probably have to have an animated 3-D engineering model and to understand and/or modify anything to do with this area. I think this is what people are referring to when they say the 9mm M&P can unlock early. If the barrel unlocks early, the barrel starts to tip upward before the bullet has exited the barrel, thus POI high on the target. This typical would happen with heavier bullets like 124 & 147gr. If shooting 115 or fast 90's brings the POI down then early unlocking might be the problem. Fixing this would take complete re-engineering of the system.

In a previous post I mentioned I had measured several barrel dimensions between a OEM 9mm and OEM .40 barrel and the only difference I could find was the width of the barrel hood.

My 9mm measures .397" and the 40 is .427" - a difference of .030."
The corresponding recess in my slide is .430", the .40 barrel has only .003" clearance and locks up tight. So, the 9mm has .033" of slop to move around in which gives me 2.5" groups at 10 yds out of my compact - not a tack driver but acceptable for social work.

I also tried approaching this from another angle. With each barrel assembled in the gun and the slide closed, I used a blade feeler gauge to measure the gaps where ever I could get in around the hood/slide fit and also between the chamber block and the left inner side of the slide.

The gap between the front of the chamber and slide was the same with both barrels (.006") and the gap at the rear of the barrel (at loaded indicator hole) was also about the same. There wasn't any front to back play. I couldn't get my gauge into the L/R sides of the hood, so instead I used my fingers to push the chamber block left and right to get the maximum lockup gap I could in both directions and measured the change in gap between the left inner-slide and the chamber block.

On the .40 it was .008" and with the 9mm barrel installed it was .017"

This was an initial quick look at ways to explain accuracy issues that seem to occur with some 9mm's and I did use a 9mm factory barrel in a 40 slide so it wasn't a matched set. You might also note that my second set of numbers don't match with my first set and you would be right. That doesn't mean were not on the right track.

Since the accuracy seems to be random and luck of the draw, I'm trying to come up with a reasonable method to check the gun with a feeler gauge in the store before you purchase it, to increase the odds of getting a good one.

All of this is just my guess of what might be going on. Measure your own barrels and see what you come up with. Sorry for the long post... already I had my nap today.