From the videos I saw the day of there was only one shot and the way Kirk moved from the impact suggests it was from the front. A suppressor would have been heard unless it was a -very- long distance away and even then you'd still hear the supersonic crack, subsonic ammo is of course possible which would make it more quiet but not invisible in the various angles and recordings of the shooting that are available. My first impression was it was rifle caliber, maybe 556 or 308. The butler assassination attempt you could clearly hear several shots on most of the cell phone footage and it matches up between sources (before the SS returned fire)