I haven't tested a whole lot yet, but it seems like Caitlyn's headshots are not consistent. In custom games, it's every 8 shots is a headshot. In ARAMs, it's 7. I played a bot game and it was 7. The whole thing changes - the passive tooltip, the mechanic and the counter. I don't know if it's a custom game glitch or you randomly get one or the other, but that needs to be consistent!
caitlyn's headshot is 8 shots at level 1, I forget at what levels but it reduces to 7 shots and effectually 6 shots. Also, shots from brush count double (so potentially every 3 shots from brush). This can attack with runaans hurricane (fires up to 3 shots at once towards 3 targets) meaning from brush headshot can activate on the main target every shot if runaans is hitting 2 peripheral targets.
If you're seeing this at level 1 without brush, then you may have a bug.
