The cam pin resembles a "T" in shape.  Since the cam pin is captured by the firing pin passing through the hole in the cam pin, I am wondering what the square flat top of the "T" of the cam pin is for?  Is it just to allow for easily aligning the hole for the firing pin?  The upper receiver has a recess on the left side to provide space for this portion of the cam pin to rest in.  I have noticed some slight wear marks on the inside of the upper receiver near this recess.  It appears that the cam pin was impacting this area, at least during the break-in period.  It almost looks like this was intended to aid bolt rotation.  I always assumed that the cam pin worked by pressure from the groove in the bolt carrier.  I wouldn't think that banging the steel cam pin into the aluminum upper would be a good idea in the long run.

Just a guess but you probably have a barrel less than 20 inches. The came pin does not usually cut the groove in an upper unless it has a shorter barrrel.

Sounds like you have seen this before.  Mine is a Bushmaster 20" flat-top.  I wouldn't call it a "groove" more like a shallow wear mark.  Any comments on my questions?

On some of the 16 inch barreled uppers I have it cuts so deep it is a groove there. I have not seen much of a mark on the 20 inch ones.