I am sorry I did not know I was talking to a Mechanic. Here is my last interaction with a mechanic:
http://www.xd-hs2000.com/xd-hs2000/Have_The_Right_Tools.html
A loose fit would allow the barrel to smack the slide.....this would peen the metal. In the gun world this is most commonly seen in a poorly fitted 1911 barrel (loose) the lugs hit the locking area of the slide and peen the locking lugs deforming them while shooting and to the extreme can break off. If the barrel lock up were loose on an XD the front edge of the locking surface would start to round off as the slide smacks into the barrel (bad). If the front edge of the barrel hood is sharp but has some nice polishing the timing is good and you may actually be using your pistol. Examples of this are the Sigs and the high milage XD`s.
A tight lock up allows the metal against metal to glide across each surface........the friction polishes the area making shiny spots. Notice I have always said a tight lock up not tight fit. The barrel is not tight in the slide........but when the barrel is locked it is pushed up into the slide by the Take down lever. This happens with all Locking tilting barrel designs pattered after Brownings original ideas. And while cycling the slide is moving across the barrel and is tight in the barrel hole opening creating all of those nice patterns at the end of the barrel.
No to the picture all tight guns will rub including your magical M&P cause it is a locking tilting barrel design. Even a 1911 will wear. Look at all of the barrels even the glock )it has around 50 round) Notice the front locking edge I see some shinies. The Sigs well they are used and abused and are all polished up with thousands and thousands or rounds. The M&P (100 rounds) It is wearing even being melonited. The Khar (not something you see every day) It wears as well but weirdly makes dark markings instead of shiny ones. The XD`s from my Tactical collection (100 rounds, 3000 rounds, 2000 rounds, 8000 plus rounds, 4000 rounds) Efk XD barrel 6000 plus rounds.
All gun barrels will be polished if you use or cycle them