I recently got a 340 M&P. Love the new carry piece. It's great. Light. Does everything my 640 Pro does except tame recoil as much. 
I decided to try some Underwood 38 Wadcutters. They've been billed as being a good round for the lightweights.
My opinion. Hated them.
Good bit of recoil, smokey as all get out at the range, not comfortable or "lighter" in recoil as I've read, and did I mention SMOKEY! I felt like it was an old western with gunsmoke everywhere.
Long and short, I'll stick to my usual Federal Hydrashok rounds I get. They work, they're accurate, and don't smoke up the range like it's a Roadhouse Honky Tonk.
Also, I didn't feel they were as accurate as my usual rounds.

I decided to try some Underwood 38 Wadcutters. They've been billed as being a good round for the lightweights.
My opinion. Hated them.
Good bit of recoil, smokey as all get out at the range, not comfortable or "lighter" in recoil as I've read, and did I mention SMOKEY! I felt like it was an old western with gunsmoke everywhere.
Long and short, I'll stick to my usual Federal Hydrashok rounds I get. They work, they're accurate, and don't smoke up the range like it's a Roadhouse Honky Tonk.
Also, I didn't feel they were as accurate as my usual rounds.