Just wanted to clear up any confusion. I already knew the answer but I decided to call S&W anyway. Just got off the phone with them, they just told me:
1. The MP on the bolt actually stands for "Military And Police" (Ok, I did burst in so sorry PainterlyShotGroup and oneyeopn, I kinda thought someone would bring this up so I just waited), however, there is some good news so the thread will not go to waste as it is a good question and discussion topic
S&W tells me the bolt and barrel for M&P15's are MPI inspected among other inspections, so the bolt and barrel of S&W M&P15's are MPI'd. If your bolt was not stamped MP don't be dissapointed because it doesn't mean MPI'd anyway.
2. The M&P15OR is a mil-spec weapon.
3. Components, parts, and materials, everything, that goes into ALL M&P15's are inspected.
Additonal information from the phone call just in case you wondered based upon something someone else posted when they posted that S&W bolts and barrels are not MPI'd and because I think I know where the original question came from:
They tell me the use of 4140 steel does not mean a weapon is over gassed or has an out of spec chamber, or has oversized gas ports. They tell me they do not purposely produce weapons with over sized gas ports or are over gassed or with out of spec chambers as any sort of cost cutting measure or otherwise, claims/implications to the contrary are not true. If the weapon is over gassed or has oversized gas ports or something is wrong with the chamber they will fix it. Mistakes happen and sometimes some may slip by as it happens with all manufacturers even COLT!
They tell me their 5.56 chamber is manufactured to mil-spec's and meets mil-spec for a 5.56 mil-spec chamber plain and simple and works fine for .223 to which you can probably attest yourself (.223 can be fired in a 5.56 Nato mil-spec chamber anyway - safety note here > do not fire a 5.56 round in a .223 only chamber). So if a weapon is stamped 5.56/223 (or just 5.56) it means the chamber was manufactured to mil-spec's for a 5.56 mil-spec chamber. They do not use any type of hybrid or "in between" chamber for a weapon spec'd for 5.56 and there is no 5.56 spec'd chamber that is purposely manufactured "out of spec" on the M&P15.
They do not list these things publically, true, and they don't send out such specific documentation or information.
I was asking in reference to the M&P15OR mostly, so I apologize for not addressing every model for the M&P15 but I did give some info that does apply to all M&P15's. I would have posted some information sooner in another thread where claims were made or implied to the contrary, and I already knew all this and its some of what i've been saying in that other thread, but i'm not permitted to disclose all the information we have to get any more specific than what you can get from them on the phone. So, I saw the post from PainterlyShotGroup and thought i'd just do something anyone can do, call S&W and get the information any person can get.
Edit note: I previously indicated here that every single bolt and barrel for all M&P15 rifles was HPT/MPI inspected. I've edited this post and am applying a correction in this note. S&W M&P15 bolts and barrels are HPT/MPI inspected, however, not every single bolt and barrel for every M&P15 is HPT/MPI inspected due to time and expense which would raise the cost of the rifle for consumers. Other manufacturers, including Colt, do the same thing and not every single bolt and barrel of theirs are HPT/MPI inspected either for all weapons they manufacture.
