First off, AR15's aren't the best option for home defense, IMO. You better hit what your aiming at, because those bullets will go through multiple walls, especially if you are using FMJ's, and if you have family members in the house w/ you, I wouldn't be using one of these. Beings you already have one and intend to use as home defense, I would just go w/ the iron sights and forget about getting a scope. Be to hard to pick up anything at that close of range.
Ammo for these is getting to be a PIA to find.
Whats your timeline for the armageddon????