You can't talk about "self defense" and SHTF. The rules for SD (and all the legal aspects of it) go straight out the window with every other law the instant you no longer have the gov't involved to enforce the laws and you certainly aren't interested in self-enforcement when it comes to your life and property. During SHTF, you'll also (I assume) be interested in defense of your property, which probably involves much longer ranges outside the home or immediate vicinity... and pure defense of property is illegal in most cases... ie, go to jail for murder.
And that's the reason SHTF discussions are generally not allowed on TFL.
IMHO, if we are talking about staying completely legal and still needing to face a jury after a shoot, and only defense of life at relatively short range... I'll chime in with a suppressed 9mm SBR... 147gr factory Gold Dots. Light, quiet, 30rds on tap. You could upgrade to .300blk with a weight penalty.