I am building a Human Gunslinger character who uses the Racial Heritage feat to get access to the Goblin Gunslinger feat so he can use a large double hack-but without penalty. Problem is, the the table for converting the damage for weapons sized for medium creatures to damage for weapons sized for large creatures stops at 2d10 and the medium double hack-but does 2d12. What damage would a large double hack-but do?
NB: the formula used in this answer was derived by reverse engineering the table for damage per size category and is not official.
The formula is 150% damage per size category - source.
If you look at 1d12, the max damage is 12, times by 150% is 18. Max damage from 3d6 is 18.
Apply that to 2d12, max damage is 24. Times that by 150% and we get 36. Smallest dice we can use to get a multiplication of 36 is 3d12, so 2d12 enlarged becomes 3d12.