It's actually very possible to synchronise any number of Pi cameras, to sub-millisecond precision if required. We do this for a machine vision task, measuring the angle of rotating disks at a very precise time (and use an IR flash which is also precisely timed, which avoids any issues with the rolling shutter).

Yeah there are a few "bullet time" demos for clusters of Pis, very cool. Does your application use network based synchronisation, or do you hardware trigger? Whenever I've built computer vision systems it's simple just to hardware trigger everything (no freerunning) and then you just readout the last frame. Almost all image sensors support external trigger inputs, but you need the breakout board to expose that pin (basically all OEM machine vision cameras provide trigger inputs as standard, but they cost more than a Pi+PiCam!).

At IceCube we have 5k sensors synchronised to within a few nanoseconds using GPS fanout and some clever clock distribution algorithms [0]. Cool stuff.

In theory if you use a bright enough flash, the absolute synchronisation of the cameras is less important. For example very high speed photography tends to rely on hardware flash synchronisation with the event. In this case you assume that background illumination is low enough that a many-ms exposure will be black and all your light comes from flash pulse which can easily be sub-ms. Provided the event falls within the camera exposure window, you don't care if there's some uncertainty. You can do (1) camera trigger, start exposure (2) delay to compensate for maximum expected jitter (3) flash triggers (4) brief delay and then stop exposure.

[0] https://www.phys.hawaii.edu/~idlab/taskAndSchedule/ARA/0810....

That's pretty much exactly what we do, we get the cameras into sync using the network then fire a precisely synchronised (us accuracy in our case, so 1000 times worse than yours :) ) a 10 to 150us IR flash to capture the specific details we're after (in our case using retroreflective targets). Main claim to trickery for us is we're worked out a way to sync the Pi cameras in software (i.e. no additional hardware needed).

Very cool to get a reply from someone working in your area, I've followed AMANDA etc in Science and Nature for many years - thanks for the comment!