Because I imagine that the buckshot is the same diameter as the small .410 bbl, so each porjective would be stablized like a slug. They would be loaded end-on-end in the hull. Guys did this way back when for their handgun shotshells. They take a block of alluminum, drill an appropriate size hold in it, then take buck shot and flatten it in that bolck so that they would match the diamter of their bore (for .45s, .38s, etc).
Hueco