Saiga makes a clip-fed shotgun.
Externally, it looks like a hopped-up AK-47, but with a standard style buttstock instead of a pistol grip. I believe it has rifle sights.
Capacity is 3 in the clip, one in the chamber, IIRC.
Personally if you want a shotgun, I'd look around for a used Remington 870. You should be able to find a beater for around $100 and it will do everything you need. With an extended mag, it should hold 7 or 8 rounds of 2 3/4" shells.
Don't let the bead front sight fool you. I've been able to achieve surprising accuracy out to 50 yards using slugs and a standard bead sight.
If you play around with patterning loads, you should be able to find a load that doesn't spread out much at the distances you plan to use it. That would mean that the dogs were as safe as with a slug. Of course, neither the shot nor the slug will likely penetrate a vest. On the other hand, a slug might pack enough of a wallop to incapacitate without penetrating...
I still vote for an SKS or AK. You should be able to get a decent SKS for just over $100.