We have 50 States, each with its own laws concerning legal hunting methods.
Nothing about ferreting is in my State's law, so I most likely CANNOT use a ferret.
Trapping is only allowed on certain animals, rabbits are NOT among them. Rabbits can only be hunted (killed with a projectile).
Furthermore, unless I am a Resident Landowner (such as a farmer) I must obtain the appropriate small game, big game, furbearer or varmint license. In either case, laws concerning "seasons" on various animals must be followed.
In a truly SHTF environment, I doubt law will really apply. In that case, any effective harvest technique would be useful.
I have a daughter. I tell her, "911 is what you dial after you're raped. 1911 is what you should have before they try."