I did make it to the range today but went to an indoor range as they were threatening rain. I had 3 other guns I needed to shoot so I had to rush the M&P shooting into one mag each through my full size barrels (.357 & .40) at the end of my hour. Both functioned fine. Both were within a 4" grouping though the .40 rendered half the mag within a 2" bullseye. This was at a 7-8 yard self defense distance.
Wish I could give you more but as I said, it was a rushed couple mags and I was slightly fatigued from shooting a couple of magnum revolvers.
One thing to think about. Usually, when one is shooting a gun with a longer barrel they are also benefiting from a longer slide which gives them a longer sight radius.
A longer sight radius is what will give you better accuracy at longer distances more so than a longer barrel with a shorter sight radius.
Hope this helps.