The 3 Shikishan Shell is not a "shotgun shell." Its closest equivalent was a case or shrapnel round. It did not come apart at the muzzle of the gun. Instead, it was fired to altitude using a time fuze that initiated a burster charge and then scattered the individual incendiaries ahead of it just like other case rounds did. It did not have a filler of "steel balls with a 50 gram explosive charge."
Here's a thread explaining the round in detail:
Sanshiki shells?