They've already revisited it once already... although still with rifles.geardog32 wrote:I may write the Mythbusters show and suggest it for one of there revisited shows.
But given the Mythbusters' track record of rather inefficient cannons, I'm sure they'd write it off as "Busted" simply based on the size of the cannon they'd need for the results to be considered fatal.
Still, I think with ice shaped into a foster slug shape and a decent high pressure cannon (probably between 0.5" and 1" bore), it would be possible to score a lethal hit that could leave no ballistics evidence - although the effective range of such is questionable.
Of course, the other issues such as the assassin (and the cannon) being able to leave the area without being identified still remain.