.40 FS slide closes when a new mag is inserted

[Jyezahn]

Though I cannot find the link to prove this statement, you can take it for what it's worth.
I came across a q&a awhile back with Julie Goloski. As I recall the question was someone having a difficult time with the slide stop. She went on to answer by saying one of the solutions is to give a bit of a tap when loading a new mag, this will let the slide close on its own. She said that it was in the design of the m&p. I'll continue to search for that link.

As for my .40 FS, it does the same. I don't recall anything about the 45 degree position though.

It's called physics. M&P is not the only gun that does this.

 

[MKT]

It is called "slam charging".

Happens when a fresh mag is seated with authority. I have had a number of duty guns do this - Beretta .40S&W, H&K USP40C, P2000 and P2000SK to name a few. Also happens with regularity with my competition S&W PPC9, and my 5906PC plus all of my competition 1911s.

Every now and then I have a reload that doesn't slam charge and I have to slingshot the slide. Fortunately, that drill has been burned in the brain so I really don't have to think about it but it is still surprising when a slam charge doesn't happen.

 

[Welderman30]

I took my new M&P .45 to the range last weekend. I noticed this happening right away and with every fresh mag I loaded, no matter what angle. I don't think that has anything to do with this. I personally like it. Not to mention how sweet this pistol shoots. Looking forward to many more fun days shooting.

 

[Magnum Mike]

My 45c does the same thing. Its a lack of excessive slide lock engagement.