I don't think theres such thing as a one gun does it all without compromise. I know if I had to go down to only one gun it would by an ar15 but I know it isn't perfect for everything. Drillings might come close IF you can get one with all the barrels regulated in say something like 308 or 06 in one barrel, 12 guage and 22lr but it would be a pretty heavy unit and the scope isn't going to be regulated for both the centerfire and rim fire, at least not if precision shooting is called for and its going to be on the heavy side. Then if it breaks down the line where the heck are you going to get parts? there very cool guns but not a survival gun. An ar15 will kill anything up to deer and even black bear in a pinch,. Has lots of capacity for a combat situation. Are very accurate out to 300 yards. With ball bullets don't tear up even small game and ammo is everywhere. Spare parts are everywhere and there simple to fix. But one gun does everything? Nope. Maybe a stripped down light ar and one of those shockwave pistol gripped short barreled 12 gauges slung over your shoulder and a 22lr pistol on your hip.
interesting post. For one gun to be used in hunting I have a 30.06 drilling, which has 12 ga barrels and 22 mag insert. The rifle is regulated 1 inch h igh at 100 yards, the 22 mag is regulated 0 at 50 yards and the 12ga barrel patterns number 4 buck or 2 shot well at 50 yards. There are other drilling rifle calibers that work great and the 16 ga is a fantastic shotgun option.
For a zombie appolyse I would go with a .308 and the platform of choice would be the M1A or the HK91.