In the first place, I SAID a 75-77 gr hollow or softpoint, and such bullets don't need fragmentation. Furthermore, if you'd shot some animals at 500 yds, or at closer ranges with lighter loads, shorter barrels, same retained velocity, you'd know that the 500 ft lbs remaining to such a 223 bullet is very effective.
As to shot placement, that's bs. When you are shooting for your life, especially with a pistol, you will be doing great if you keep half of your shots somewhere on the chest. The heart is the same size as the brain. If you could reliably hit the heart, why not just hit the brain, and get the instantaneous effect you need (but won't get with a heart hit)?
No, the Makarov's 220 ft lbs is not adequate. It's not even close, except in the same way that the .22lr is "adequate" (ie, as a "bluff", when you don't actually have to fire in order to make the attacker back off). The further away the guy is, the less need of an immediate stop. There's no reason to care if a guy flops around a bit, after being shot, at 300+ yds, because he's no threat to you at such a range (after taking a decent hit). A .45 sized hole, relative to the size of the vitals, doesn't mean a lick of diff (as vs a .22 sized hole). What matters is the tissue destroyed beyond the crush cavity (if your bullet has enough velocity to deliver such performance).
The 223 has plenty of power at the ranges where an immediate stop is needed. For shtf, why lug around more weight and bulk, in rifle, mags, ammo, and be without the GI gun, parts, mags, and round? Mobility and stealth will be hyper-important.