Ditto Legionnaire's pick. My NAA .22LR (even less useful than his .22Mag) was irresistibly cute, and was my token carry gun at my wedding, and strikes me as the most useless gun I've got:
- .22LR out of a 1.125" barrel can't do enough damage to be anywhere close to useful
- inaccurate
- slow to cock/aim/fire
- to small to operate & aim well.
For the basic rule "have a gun" I won't even carry that. As Cooper noted "if carrying one comforts you, do so - but do not load it, for if you load it you may fire it, and if you fire it you may hit someone, and if you hit someone you will give them cause to do you great harm."
Next in line is the extreme opposite end of the pistol spectrum: a Mossberg 590 AOW - a pistol-grip-only shotgun with 14" barrel. A must-have raspberry for anyone living in NY (the look on the county clerk's face when registering a 12 gauge pistol is priceless, and the sheriff had never in 13 years had someone ask him to sign NFA paperwork), but aside from the legal novelty it's too big to carry, too much kick to safely aim properly, and generally not useful for anything other than extremely close combat bordering on contact distances.