I am selling my 308 MBR, having realized that a shotgun is infinitely more useful. For hunting, you can have a variety of loads, ranging from slugs to #8.5 for smallest game. I really liked #2 with Flite Control Wad, tight pattern out to 40 meters. The load is called BlackCloud, good stuff for turkey or whatever.
So if TSHTF, you are set for hunting. I have a much easier time hitting something with #6 or #7.5 versus a 22 LR. Plus the cost of 22LR has escalated so high that #8.5 is not that much more expensive but throws a gazillion pellets if you hunt small game. You can find deals on #4, #6 birdshot. etc.
For tactical usage, with an extended mag, you have about 9 in the mag plus 1 ghost loaded and 1 in the chamber. That's 99 pellets an experienced shotgun person can empty in a few seconds. Some of these guys on youtube are scary fast, they got their VersaMaxes going at nearly full-auto speeds. No submachine gun or AR15 can match that at CQB distances. Plus there is no muzzle rise, each 9 pellet pattern goes into the same spot. Vastly increasing hit probably on several moving targets. This is what TSHTF might be, several moving (and armed) targets and the distance is measured not in meters but feet. So an AR15 with 500-meter hit capability may not be the best for room-clearing. I hear the most experienced room clearing people take something like Benelli M4.
A slug can stop a car engine and buckshot can target the tires if a moving vehicle has to be stopped. Much more difficult to stop a vehicle with a 9mm or 223. Maybe even impossible.
I noticed during the ammo panic last year, the shelves were wiped out except for 12 and 20 gauge, with some 410. There was literally nothing else available. Even a 20 gauge is considerably more powerful than 44 Magnum, with a higher hit probability. So you should be able to always obtain some kind of 12 gauge ammo. It's universally desirable. Even #4 buckshot is a serious defense mechanism. With a 3" magnum shell, you shoot 15 pellets of 0.33 caliber, that's like emptying the entire magazine of a 32 auto with one shot. A 3.5" magnum shell of buckshot holds 18 pellets.
With 12 gauge, Flite Control Wad doubles the distance to about 40 meters. Cheap buckshot from Walmart really sucks. The patterns all go to hell at even 25 meters. FCW is much tighter. At least 1/2 the size. I could even obtain hits at 100 meters with Federal FCW buckshot but it was of course questionable at that kind of distance. At room distances, FCW is pretty tight, the size of a large dollar coin.
Glocks are always backups weapons. Bringing a Glock to a shotgun fight is as smart as bringing a knife to a gunfight. I had a acquaintance who fought in an African war in the 70's. He said, as a combat vet, he considered pistols nearly worthless and would prefer having an extra mag of ammo for his FAL rifle. Pistols are just the last resort, oh-shyte backup when all else has failed. If all I had was a Glock and I ran into an adversary with a 12 gauge, my first reaction would be to run as hell.
An AR15 rules outside of the 50 meter range of course. Another huge disadvantage of shotguns is they are difficult to supply with ammo, it's bulky and heavy. 200 rounds of 12 gauge buckshot weights 25 lbs, even though it has 1800 pellets. Plus the longer the ammo tube, the longer the shotgun. So a 9-round capacity shotgun is kind of huge and bulky compared to an AR15 with a 16" barrel.