My father has a 9mm pistol and a Kel-tec 9mm carbine. He kits them together along with other supplies has his grab-n-go SHTF kit. He figures that he would have two weapons with a common ammo and then just carry more of 1 ammo type instead. Also he still has a 12 gauge shotgun loaded with #4 has his primary home defense weapon.
With additional time to prepare, also to be dragged along are several other rifles/pistols of various calibers.
My grab bag contains a .357, and 12 gauge. As funds allow I will be adding an AR15 to the mix. My HD weapon is chambered for 9mm Mak, and would also be tossed into the kit if we had to leave in a hurry. The Mak might seem like a 'weak' caliber to some, it will still work for HD without tearing through my house completely.
Given 15 minutes extra, additional weapons and supplies would be loaded up into the vehicles to go.
But I think any SHTF kit does need a semi-auto rifle with at least modest magazine capacity.
CZ82, S&W Mod. 65 .357, Marlin 22mn (22WMR), Mossberg 500.