So, best case, the AA-12 Combat Shotgun. 30 round clip, limited recoil, and the ability to switch ammunition types on the fly (12 gauge scatter, 12 gauge slug, High-Explosive Grenades, Anti-Vehicle Grenades, and Frag Grenades).
...with it's rate of fire you could theoretically unload 120 High-Explosive grenades on target.... in one minute.
http://www.youtube.com/watch?v=p4ebtj1jR7c
However, if you're talking about backup weapons I actually have in my house? Probably one of my swords (since my lightsaber still is made out of plastic, despite my half-hearted efforts) or my 15 pound sledge.
//J.