Okay, guys, this may be a tough one. My M&P .40 - 5.5 is just great. I have put a lot of rounds through it, store-bought and my reloads.
I had a new bag of WW brass, so I started to make up a box with 155 gr. Hornady STX bullets. I used 7.1 gr Auto Comp (7.0 min. -7.8 max) and loaded to spec length. I have tested this load before with no problem. After retrieving spent brass I saw some splits just at the neck and turned INWARD. Checking other cases, there seemed to be a small nick right at the neck and slightly flattened.
Comparing to other factory loads, I decided that crimp was not as tight as some, so I put some more crimp and tried again. No more splits, but there is still a small nick and a little flattening in fired cases.
So, I took 3 factory types and 4 reload recipes (including the tighter crimp) to the range and ran some of all. That small nick (sometimes almost unnoticable) was in almost every fired brass, right at the neck, and tells me that the brass is hitting something when ejected.
I repeated this procedure with my XD-Sub compact, and there was no hint of any problem.