RIKA, "Mount your lover?" Don't tease

Gripper, I like you, but you are just plain scary sometimes (though not nearly as scary as Melvin).
Topic specific:
If I had to choose one and only one firearm for major SHTF, it
might be a shotgun. As was pointed out, in CQB it is especially effective. Outside of CQB range, the prudent person will seek to evade rather than engage. It might be hard on a slug barrel to fire shot through it, but it can be done. Therefore, a shotgun with a rifled or smooth bore slug barrel will handle most any ammo available for that shotgun's gauge. This makes it at least a passable hunting weapon as well as a CQB gun, provided the barrel is short enough to be used effectively inside structures. Yes, I know that the shorter barrel would adversely affect accuracy at longer ranges. It is a trade off.
I have seen fairly fast rates of fire with breach break shotguns, both single and double barrel, in the hands of a good shotgunner that is experienced with that particular weapon. That said, there is no way even the best shotgunner is going to achieve the rate of fire with a breach loader that can be achieved with a pump or auto-loading shotgun, much less a semi-auto rifle or handgun. Moreover, a breach loader is totally unsuitable for CQB. You simply would have to put shells in it too often.
Like I said, I might choose a shotgun if I was forced to choose only one firearm to carry into SHTF. In reality, I would likely choose 5 or 6 weapons if I had the choice and wherewithal to transport them if necessary. They would include a shotgun for CQB and possibly some game hunting, a 10/22 for small (and maybe not so small) game hunting, a center fire rifle (my preferred round is .30-06) for larger game and long range sniping if I felt an unavoidable need for it, an AK, SKS, or AR (if the other two were not available) for intermediate range and possibly CQB, and an autoloading handgun with a revolver backup. That is six. I have those weapons available now, whether I would have them in SHTF is a matter of where I am when it hits and whether I can carry them all with me if I have to bug out. My assault wheelbarrow and tire raft are currently in need of repair although I do live right on the Ohio River. It is less than a mile as the crow flies.