The newer Lyman 'sabot' slug, that fits inside a normal Red WW field wad (and others) is very much worth a try. Size-wise, it is far superior to normal sabot encased slugs as it's 14 bore (.690") in size. Since the nose if very blunt, it will deliver excellent 'punch' on impact, yet in media, they penetrate very well indeed.

Of course these slugs, with their plastic wads, will plastic foul a barrel if shot over black powder. They need good protection between the plastic wad and the powder.