Impracticality

So the subject today is impractical but cool guns. Odd question, easy answer.

Any manually operated Gatling gun. Bonus points are given for examples in a historically accurate caliber like .45-70 or a scale model in .22 LR. Unless you really are one of the extremist wackos we've been warned about and have set up one to cover the entrance to your bunker, they just aren't all that useful for anything. They are, however, exceedingly cool and I want one.