Charter Arms Bulldog .44
In all fairness this was 30 years ago. Blasted thing was so inaccurate you couldn't hit a 4 x 4 foot piece of butcher paper at 10 yards with it. I worked with the mfg for a year and a half - never got one bit better. Tried custom reloads, factory ammo, everything I could find. It was shaving lead, but no amount of work seemed to help. Even the factory was stumped, if you can believe that. I finally got a full refund from my great dealer and the thing went back to Charter. I presume it is mounted on a wall somewhere there to remind someone of the need for quality.
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Worst gun I've never owned? Easy.
Smith & Wesson Sigma 9mm
My daughter bought it and despite numerous trips back to S&W, it never got through as many as 4 rounds without a stovepipe. Finally traded it off at a gunshow.