So your players have set a wagon of gunpowder on fire...

[Thelemic_Noun]

This rule can also be used for things like thunderstones, flash powder, or alchemist's fire.
Find the Strength score for which the charge would be a heavy load. That number times 1d6 (plus 4d6 if it is gunpowder, or 6d6 if it is flash powder) is the damage dealt (Reflex half, DC 10 + ½ equivalent Strength score, +4 if gunpowder, +6 if flash powder) within a 10-foot-radius (+10 feet if gunpowder, +15 feet if flash powder).
For every 10 points of Strength, the radius increases by 10 feet.

[Young0ne2]

The World Of Warcraft RPG has rules for just such an instance.
<Warcraft- The Role playing Game, p. 179)
Gun power: 2d6 fire dmg to all characters in a 5ft radius around the blast point ( DC 18 reflex for half dmg)

for every pound +2d6 and 5ft to the total.

Example. 10 lbs.= 20d6 & 50ft Radius.

"Imbued gun powder" that is gun powder with a bit more Alchemy in it, does double what the regular gun powder does.

[Lamontius]

have they also been drinking heavily with no shirts on
because I feel like you are describing my player group

[Pyromaniac]

... go on....

[blahpers]

Thelemic_Noun wrote:

This rule can also be used for things like thunderstones, flash powder, or alchemist's fire.

Find the Strength score for which the charge would be a heavy load. That number times 1d6 (plus 4d6 if it is gunpowder, or 6d6 if it is flash powder) is the damage dealt (Reflex half, DC 10 + ½ equivalent Strength score, +4 if gunpowder, +6 if flash powder) within a 10-foot-radius (+10 feet if gunpowder, +15 feet if flash powder).
For every 10 points of Strength, the radius increases by 10 feet.

I like it!