I've got a weird corner case planned out in my head I want to share. An Asura-spawn Tiefling with a lucky roll on the Fiendish Heritage table can get the ability to 'use Large weapons without penalty'.
Does this let them use siege firearms as 2handed firearms? "A Large or larger creature can use a siege firearm as a two-handed firearm", and the Tiefling isn't large sized, but acts as one in ways that are relevant in this scenario.
The other question is obviously whether it's worth it to even bother. Shot and powder costs 10x as much per shot, and the cannon costs 4x as much as a musket. You're probably going to also have to invest in a handy haversack or bag of holding to carry the cannonballs/powder in, as well as Muleback cords to carry the damn thing at all. Even then, you take a -4 to hit.
The benefits over a standard muscket are 6d6 damage instead of 1d12, 100 foot range increments, and the ability to use scatter shot for a 60ft cone. You can also use the special siege weaponry, which does interesting thing like 30 foot blast radius explosions or entangling a similar area in ice.