I think I have identified the source of confusion here. The big guns
on the Yamato are referred to as 'the main battery'. This is a term
stemming from the old sailing ships. So when you hear someone casually
mention 'the main battery fired antiaircraft shells' one unfamiliar
with the term might assume a battery-activated antiaircraft gun. BTW
those San-Shiki shells were used but weren't effrective on aircraft.
They were also to be used to destroy merchant shipping should the case
arise. I suspect they also may have tumbled out of their racks and
detonated when the Yamato rolled onto her beam ends and blew the ship
in half. FWIW Bismarck also fired her main battery at the Lancasters
coming in to sink her on that last mission - but to no avail. Wonder
what it was like when the Tallboys came down. Richter 9?
Walt BJ