That image is not high enough quality to say that.
There is a noticably bloody area in is right upper earlobe that could be a chunk taken out by basically a 5.56 round, but again, its too crappy to be definitive.
Also this twitter poster claiming it is a bb gun is a fucking moron.
BB Guns do not have have an echoing report after the initial shot, as is easily audible from the shots that occur after the audio re equalizes when Trump goes down from the podium.
The audio sounds tinny and poppy because it is equalized for human voice in a large outdoor environment. A BB gun would not be audible whatsoever.