After visiting the range today, I disassembled my M&P 15 to clean it and just after separating the upper from the lower, I noticed a small ( 1/4" x 1/8" ) black plastic plug, cap, insert, or grommet fall to the floor. I didn't pay much attention at the time but on re-assembly I couldn't find where this part came from or where it should be put back to. I have checked several sources and diagrams all with out any success. I don't think its of any consequence...but, I am seeking any advice or suggestions.

Is the rifle stock? If it is, what model is it? Certainly doesn't sound like anything I've ever seen on any mil-spec type AR.

Rifle is a stock M&P 15.

I have an M&P 15 and have taken it down numerous times for cleaning and education. That does not sound like a part I have come across. Can you upload a photo of the part please?

Not sure if it is the same on a full size AR but I have a 15-22 with what sounds like a similar cap. Mine fell out of the buffer tube, I noticed it when I installed a Slide Fire Stock. The cap was on the ground after several full mag dumps. After it fell out twice I decided to keep it off so I do not lose it.