Im going by my best guess at a wound size via images, and its currently being reported that the shooter was outside the event perimeter, meaning the shots would have been taken from likely outside the effective range of a pistol, so, while it is still a guess, I am still going with a 5.56 rifle of some kind.
Also, in my experience, common pistol rounds barely make any report as can be heard in the audio in similar conditions (large open area).
The gunshots sound tinny and plinky because the audio was set up for a speech in a loud noisy environment, watch similar audio footage of say a reporter in a warzone and you will find distant rifle rifle sounds similar.