So in a SHTF scenario, I would think that most people getting shot at would rather relocate to other places than check the caliber of the incoming fire, to see if they want to press the issue.
The way I see SHTF scenarios is that you will be defending an area, either your property or a vehicle you use to move away with. In either case, noone will just walk up to you and ask for change, and the presence of any kind of firearm will be a deterrant. And if you have lots of ammo(that's easy to carry), you can lay a pretty good suppressive fire.
Seeing this thread made me google up some .22 machineguns, both drum fed and belt fed, and I think anyone being met by any of those would turn around pretty fast.

But I dont feel I'm very qualified to discuss pros and cons of different calibers.
My experience is that *any* gun will make most attackers run, be it a shotgun or a rifle. But that's just my limited experience.