Its sounded like a .22 to me, but its hard to tell having not been there, also the way people described the sound reminds me of a .22
Discussion
22's don't normally blow dudes heads open. Brain matter on the bleachers.
I didn't see photo of person in crowd just the blood on the bleachers
Interview with doctor at scene said dudes brains were blown out
And a .22 would be the last gun I'd bring to an assassination.
Could be a higher powered rifle, just didn't have a loud noise on the videos I saw, also I'm not sure the proximity of the shooter and idk whether or not a suppressor was used. A high powered rifle even at quite a distance usually is immediately recognizable as nothing other than gunfire