No......
a loose cannon that's firing is actually more likely to do damage to the ship or fort it's firing from than it is to do damage to the enemy. That's precisely what the phrase means. Think it through from the perspective of single shot cannon in the era of sailing ships, which is where the phrase comes from.
Unless your intent is simply destruction all around.
Step off. You know what I meant. First of all, I don't really believe McCain is a loose cannon. If he is, at times, then it ain't necessarily bad. It keeps our enemies guessing. What's more, you left off the second part of my cannon comment, that a firing cannon is better than one sitting quiet. There is no other potential candidate out there, other maybe than Rudy, who I trust to actually fire the damned cannon.
#294, thanks for explaining that so well.