Well, power is NOT a concern...
If you're worried that a potato cannon won't take someone down, then you've never seen ME load one!!
My best shot, in our Scene Shop/Weapons Lab (and I have witnesses, one of whom is my wife (this will become pertinent, later in the story!!)) was dang near fatal!
We had built one with a 5' barrel, and had been experimenting, essentially re-inventing the gun, with it all evening. We had discovered little things like wadding and compression, but were still just '...not there, yet.' We weren't sure where we were headed, but we'd know when we got there, we felt confident. Anyway, my girlfriend (who had been drug along more as a safety check than anything else) had disappeared, so I figured now was my chance! I loaded a potato, wadded with newspaper, then a second plug, wadded with (and this is important!) WET paper towelling. Then I sprayed THREE seconds' worth of B12 (almost a triple powder measure, by the evening's standards) and locked down the breach.
We took aim at our target (a sheet of 3/4" ply at the far end of the shop, some sixty feet away) and spun the Coleman igniter that was the firing mechanism. Then, several things happened very quickly: first) the gun went WHOOMP!, second) it leapt out of the hands of three very shocked stagehands, clattering on the floor somewhere behind us, third) there was an almighty crashing, and fourth) we heard screaming.
Upon further examination, we found that I had created the equivalent of a tactical nuclear railgun, loaded with tubers! The potato APFSDS rounds had penetrated the plywood...and the set-piece behind it (knocking it over--that was the crashing)...and the metal lockers behind that...and the partition wall behind that, finally coming to a splattering conclusion again a cinderblock wall, some two feet above and to the right of my girlfriend's HEAD, where she sat on the toilet, in the Ladies' room.
DOH!!!!
We promptly disassembled that gun, trying hard not laugh anymore (it was hard, through tears of laughter (and for me, pain)). And the amazing thing is, that girl MARRIED me!!
Bad judgement? Guardian Angel Complex? Drunk? That's for you to decide. The moral of this story is...guns don't scare the sh!t out of people...PRODUCE does!!!