In Pathfinder Player Companion: Dragon-slayer's Handbook
it states under the Wyrm Sniper Archtype
"Artillery Proficiency: Wyrm snipers are proficient with light siege engines, in addition to the normal gunslinger weapon proficiencies."
I have looked all over and cannot find an example of a "light siege engine."
Their are some Siege engines with light in the title. But that answer feels a bit lazy to me, and I find it hard to believe that a Gunslinger siege archtype doesn't even allow them to use something like a cannon if they could only use ones with light in the title the only gunpowder operated one they could use is the light bombard.