How about a variation on suggestion #2... Slugs work normally (like powerful hunting rifles, as HellHound said). When using shot, however, the attacker targets a 5'x5' square. He makes an attack roll against AC 10, to hit the square. Anything within the square takes full damage, unless it makes a Reflex Save DC 15 for half damage.
Just like autofire, but smaller and it only uses one shell.
It makes it easier to hit a single large target, or you could hit several closely grouped small targets (Tiny creatures standing in the same square). And it's the sort of weapon, your average Joe could reliably hit something with at short ranges. [/B]