So I had an... interesting realization. Cannons are unwieldly and take multiple full round actions from an entire team to reload, but are very powerful. It's the reason gunslingers only use them if they're going after something like a fortified gate. However. We're looking at 6d6 per shot, which is honestly pretty good for actual weapon damage, but that's because, you know, it's a siege weapon
Here's the thing though. The Dead Shot deed allows you to pool all your iterative attacks into one singular massively powerful shot. a level 11 gunslinger with a haste buff and the rapid shot feat is looking at a single cannonball dealing 30d6+(x5 dex) damage. 40d6+(x5 dex) if it's a fiend's mouth cannon, and really, you want it to be if you're already dumping thousands of gold into this strategy. Plus, if you manage to critical with even one shot, you have a very good chance of quadrupling the damage from the whole shebang.
Oh, yes, and as a little extra gravy, the damage is dealt as one lump sum, so Damage Reduction? What's that?
Let's take a level 20 gunslinger for a moment. If he has rapid shot and a haste buff, he technically has seven iterative attacks. Assuming he's using this with a fiend's mouth cannon, that's 56d6 damage.
We can assume he has a minimum of a +10 dexterity bonus, which is quite frankly not all that large for the level it's at, which bumps the damage up to 56d6+70.
Assuming he's also taken Improved Critical (Cannon), he has a 10% chance per hit to make it a critical hit, which on such a weapon is x4 damage. If you do the math, that's a 52% chance to critical hit with this thing, so you're looking at 224d6+280 damage.
This sets the damage at 404 damage minimum. 1064 damage on average. 1624 at most.
Assuming I didn't miss something, this makes a gunslinger with a bunch of cannons he can pull out of a bag of holding one of the highest direct damage classes in the game. I've been known to miss glaringly obvious things though, so if someone catches it, let me know. Even if it doesn't get a critical hit, that's still an average damage of well over 200; high enough to turn most enemies into paste, or at least leave them feeling like they shouldn't continue fighting anymore.