My M&P 40c came with the medium size back strap from the factory. IMO, S&W did a better job on making the M&P series back straps more easily interchangeable than did Springfield on the XDM or Glock on their Gen 4's. It's a pain on my XDM--I actually broke a brass punch doing the back strap swap out the first time. I think it's a bit ridiculous when you need a mallet and punch to do that. I take my wife and kids to the range and I'm glad they can shoot just about anything of any size rather well, w/o me having to change grips, stocks, etc. We'd rather bring our sights already zeroed in and our magazines preloaded, so we can spend our time shooting!