@ FlemGem
Autoloading shotguns are excellent for crowd control and close quarters. Their theoretical and practical rates of fire far exceed pump-action shotguns. Many autoloading shotguns can accept specialist ammunition such as slugs and sabot slugs in addition to the universal 00 buckshot. Good luck getting your hands on incendiary shells or the new explosive "grenade" shells.
Shotguns are good for inexperienced shooters but their internal tubular magazines make reloading a pain in the ass and shotgun shells are not only bulky but also heavy. 20 rounds of 7.62x51mm NATO takes up the same amount of space as 6 shotgun shells. Reliably and maintenance can also come into consideration, especially in an apocalyptic scenario. Most shotguns, particularly semi-autos do not field-strip for all-but-mandatory cleaning easily or quickly and can be confusing for an inexperienced shooter, whereas weapons like the M14 and M1911A1 field-strip in seconds and seldom need cleaning.
If you DO decide to get a semi-automatic shotgun, I would strongly recommend the Remington 1100. It's the most reliable semi-automatic shotgun in the world. In 1978, a Remington 1100 fired 24,000 consecutive shotgun shells without stoppage, malfunction, breakage, or cleaning. If you can't find an 1100, the old Browning Auto-5 (the *first* semi-auto shotgun in the world, designed by the greatest gun designer who ever lived, John Moses Browning (M1897 Trench Gun, M1911 pistol, M1918 BAR, M1917/M1919 machine gun, M2 heavy machine gun)) will still perform very well.
Personally, I'd take an 1100 over just about any other shotgun in the world, with the possible exception of the M1897 Trench Gun (because of its bayonet lug slamfire capability, and legendary reliability) but good luck finding an original USGI M1897 in this lifetime
I will reiterate, the M14 is the way to go.