If regulated for slugs, it may be only for the older-type Breneke slugs.
: As above, try many different makes, weights and types including any Brenekes but probably not sabots. One may work closely enough. Trying them is the only way.
: Modern slugs are either undersize and/or have vanes on them that collaps to prevent damage to chokes.
: The older drillings I'm familiar with (locally owned)were often used for driven wild boar with Breneke slugs.