Frag-12 rounds are like small grenades, which sounds pretty nasty. Dragon's Breath, on the other hand is pretty ineffective, realistically speaking and tends to ruin barrels.
SPAS-12s are notoriously unreliable because they're so complex. I'd take a Benelli. Realistically, for all the bulk and stuff, a AA-12 still loses to a grenade launcher or machine gun.