They designed that tear-drop shaped bevel below the striker hole in the breech face to help mitigate the potential for shaving primer cups (whether plated or unplated).
Such shavings can occur when the barrel drops and drags the case down the breech face, and a sharp edge of a cleanly cut fir pin/striker hole might shave some metal filings off the primer cup. Obviously, an accumulation of such metal shavings and debris might interfere with normal fp/striker movement inside the slide and create the potential for a light-strike.
It's not uncommon for primers to expand a bit when the cartridge is fired, and in the M&P's this can cause some minor flow into the machined bevel below the hole in the breech face.
You ought to look at some fired cases of ammo used in Glocks, with their rectangular fp hole.
It can look a bit startling if you're unfamiliar with the design and how it normally functions in this regard.
A little pin "wipe" isn't unusual, too.
FWIW, I've also seen how Glock started to give their rectangular fp holes a bit of a mild bevel over the years, and I suspect it's for much the same reason, being to mitigate the potential for metal shavings off the the primer cups to accumulate. Of course, Glock also has a "debris hole"on the bottom of the slide, right behind the head of the fp channel, which is intended to let debris drop from inside the slide at that spot to help prevent an accumulation that might interfere with fp movement and impact force.
(Yes, Glock calls their striker-type firing pin a "firing pin", and S&W calls theirs a "striker". Same thing.
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Shield - Chipped chamber face by firing pin hole
Linear Mode
