I think what you are seeing is the chin of the guy in the background
Also why I am not sure it is a recoil boot is that it curves toward the comb of the stock and there appears to be a vertical line at the end which makes me think butt plate.
I noticed no straps on the shoulder of the marine in question and would expect something to carry rifle grenades etc.
Guess we wont know unless there is another pic in the series