Pathfinder Damage Dice Steps

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?

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Fair warning (as well as for anyone looking to reuse the concept), your GM might veto the idea because it doesn't fit RAW or thematically. Goblin Gunslinger allows you to use Medium firearm weapons with no penalty and its unlikely training with small creatures would help you use Large weapons because they still wouldn't be able to. – Ifusaso Apr 11 at 2:01

There is now an official table and rules for adjusting weapon damage across the whole scale. Those are available in the Paizo FAQ. (Announced just a few weeks ago on the Paizo boards.)

Size Changes, Effective Size Changes, and Damage Dice Progression: I'm confused by how to increase and decrease manufactured and natural weapon damage dice when the weapon's size or effective size changes. There's a bunch of different charts, and I'm not sure which to use.

When the damage dealt by a creature’s weapons or natural attacks changes due to a change in its size (or the size of its weapon), use the following rules to determine the new damage.

• If the size increases by one step, look up the original damage on the chart and increase the damage by two steps. If the initial size is Small or lower (or is treated as Small or lower) or the initial damage is 1d6 or less, instead increase the damage by one step.
• If the size decreases by one step, look up the original damage on the chart and decrease the damage by two steps. If the initial size is Medium or lower (or is treated as Medium or lower) or the initial damage is 1d8 or less, instead decrease the damage by one step.
• If the exact number of original dice is not found on this chart, apply the following before adjusting the damage dice. If the damage is a number of d6, find the next lowest number of d6 on the chart and use that number of d8 as the original damage value (for example, 10d6 would instead be treated as 8d8). If the damage is a number of d8, find the next highest number of d8 on the chart and use that number of d6 as the original damage value (for example, 5d8 would instead be treated as 6d6). Once you have the new damage value, adjust by the number of steps noted above.
• If the die type is not referenced on this chart, apply the following rules before adjusting the damage dice. 2d4 counts as 1d8 on the chart, 3d4 counts as 2d6 on the chart, and so on for higher numbers of d4. 1d12 counts as 2d6 on the chart, and so on for higher numbers of d12.
• Finally, 2d10 increases to 4d8 and decreases to 2d8, regardless of the initial size, and so on for higher numbers of d10.

Damage Dice Progression Chart

1
1d2
1d3
1d4
1d6
1d8
1d10
2d6
2d8
3d6
3d8
4d6
4d8
6d6
6d8
8d6
8d8
12d6
12d8
16d6
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Im honored to have been awarded the bounty. – Fering Jun 2 '15 at 3:19
The chart is correct, and I do not dispute the answer. However, am I correct in thinking 2d12 (13) made a step smaller actually does more average damage at 3d8 (13.5)? 2d12 = 4d6 -> 3d8. – John Rhoades Sep 1 '15 at 16:03
It would appear you are correct. However, such a weapon would go down two steps, not one, to 3d6 (10.5). You are correct that it seems ridiculous, however, and the 2d12 is more comparable with the 3d8 step in the first place. – Eagle0600 Feb 12 at 2:53

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.

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Pathfinder seems to use the same table as D&D 3.5.

This conclusion is from the fact that available sources (this and that) do not contradict that 3.5 table, but rather seem to be a subset/superset. The underlying math looks like this.

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