Would the AR-15 be a good rifle for a SHTF survival situation? The simple answer is, “it depends.” The M16 223/5.56mm was original developed as a replacement for the M14 and the 308. The mindset was to develop a lighter rifle and lighter ammo so that soldiers could carry the rifle further and carry more ammo. One drawback, instead of shooting a 150 or 180 grain bullet like the 308 Winchester / 7.62 NATO, the 223 / 5.56mm shots a 55 grain bullet.
1. To compensate for the smaller bullet diameter and lighter weight, the 55 grain 5.56mm is supposed to “tumble” after it hits flesh. The “tumbling” creates wounds and does quit a bit of damage to flesh.
I see a couple of issues with the “tumble” theory:
How does the bullet know whether its hitting a wall and should punch through, or flesh and tumble? When the bullet is made, is it granted with magical powers that tells it what the bullet is about to hit, and whether it should tumble or not?
All phun aside, the 223/5.56mm has problems getting through walls. Once the bullet hits something, and it tries to tumble, it loses its energy.