I purchased my 4" in March after shooting my buddies 4 5/8" that I liked very much. I love the ergonomics. We both started developing failure to feed issues and sent them back. They both came back with a note that the ramp was polished. I was skeptical, but it does seem a lot better.
Last week I had what I thought were several light primer strikes. I then realized that it wasn't going fully into battery. In fact, if I point it upward, the spring is not strong enough to get it into battery.
I called S&W last week and told them about the issue and that I had read it may be under sprung. They told me that the guns are now shipping with a different recoil spring/control rod assembly and were going to send me one. Unfortunately, they sent me one for the 4 5/8" barrel. The label says it's the assembly for the M&P 45. I'm assuming it's stronger.
Anyway, after calling back yesterday, I received a voicemail today that they are sending me the correct one. I'm guessing it's the M&P Compact 45 assembly. Fingers crossed that this fixes the problem.
Also, I've only shot target ammo and some HSM Bear Load ammo. Nothing crazy. I'm a bit annoyed that I've spent more on ammo trying to get this gun to work than I spent on the gun itself. Hopefully this takes care of things and I can start playing with some hotter stuff. I do really like it, but it needs to go bang.