Re: downrange?
The .338 would work, but the .50 cal would work even better. Problem would be getting the spark needed and with the videos running, tracer or incendiary is not an option. At one point there were some .50 HE rounds but I think those were only experimental. But you're right on the target A/C... there's probably minimal sweet spots.
If we're looking at conspiracy, 3 weeks between the explosion and the "visit" would allow someone to at least pick up any empty casings and/or beer bottles <jk>.
However, as I recall, the strut failure on the previous big boom was detected by sound sensors. I would think they would have been operational during fueling if for no other reason then to ensure they worked. I do believe they would have picked up the sound of a bullet impacting the rocket.
I wish Musk, et al, lots of luck on sorting this one out