Looking at the REAL ballistics of the CAR-15 with an 11.5" barrel, seems to me that you might as well pack a 12ga shotgun loaded with military grade flechettes. You also have a much higher first shot hit probability per shot at the ranges that 11.5" CAR-15 is good at.
For use in pre-SHTF times, the 11.5" barrel on the CAR-15 is illegal unless federally registered, and the AR itself is just illegal and restricted in many jurisdictions.
A shotgun is legal pretty much anywhere in the USA and in most of the world for that matter.
Now, while flechette ammo may not be legal everywhere, the flechettes are loaded into commercial cases and are indistinguishable from normal ammo.
As for conceilability, a shotgun can also be easier to conceil than an AR by a significant margin.
There are some with silencer fantasies, where the silencer is the 'magic wand' that always ensures victory, but this is nonsense. Yes a shotgun is hard to silence, but you get pulled over with a shotgun in your trunk, and it's no problem. You get pulled over with a suppressed 11.5" CAR-15 in your trunk, it's a BIG problem.
M1A7 Sabot Muzzle Velocity - 1925 ft/sec
Single Flechette Muzzle Energy - 66.17 ft/lb
Flechette Cluster Muzzle Energy - 1257.23 ft/lb
Single Flechette Weight - 8gr
Flechette Cluster Weight - 152gr
Packing - 19 flechettes per 2-3/4" shell
Point Target Range - 75m
Area Target Range - 150m
Max Effective Range - 300m
Digitized M1A7 flechette sabot ammunition target groups fired from supported rest using a standard issue Mosberg M-590 shotgun with 20 inch barrel. Targets used are standard U.S. military E-target sillouette, gross dimentions 48 In. x 48 In., range 20 Meters/21.87 Yds./65.61 Ft., aim point marked X. The M1A7 Flechette sabot ammunition outperforms all competing flechette and buckshot for shotguns in grouping, effective engagement range, and terminal performance.