There was a time, a few years ago, whenI thought the ideal SHTF weapon was my Remington 700 in 300 Winchester Mag. This is a true 1000-yrd rifle and it would do a great job at a long distance poke and then let me run like hell....
These days, however, I firmly beleave that the best weapon is without doubt the AR-15. I got into shooting DCM/CMP and I will admit after shooting full cource at 600-yards and printing groups that indicate that I could engague human sized targets at that distance combined with the rapid fire capacity, cheap and plentiful spare parts, cheap high capacity magazines, cheap and easy to carry 223 ammo, and the abuility to have various uppers, stocks, grips, accessories to customize the AR-15 for any purpose makes the AR-15 THE SHTF weapon of choice for me!
Now, I also recently purchased an AR-10 that I hope works out nearly as well as the AR-15s so I also have a 308 but the AR-15 is the first weapon of choice.... Well, actually I like my ARs so much I bought an M-16 and if the shit really hits the fan, I will be using the M-16 to come to the aid of my country!