Just as a nitpick: a part of any automatic weapon's design is the cooling mechanism (even if just the appropriate amount of bare metal) to keep the barrel from overheating. A successful Force 11 Ignite spell will have greatly overcome the cooling mechanism. A red hot barrel is step one of a very nasty surprise if you keep firing.

You're still thinking about ignite as a shroud of fire applied to an object. That's not what it is. It's kenetic agitation until the object itself is so excited that it begins to combust. By its very description, the material in question, is on fire. It's not a fire applied to a target, but a target which has gotten so hot, that it bursts into flame. The material is the source of the fire. I guess you could compare it to the mutant power of Gambit of the X-Men. The mage excites the object's molecular structure, like some kind of psychic microwave, up to the point that it catches fire and ignites nearby combustibles.

Also, a citymaster would need at least a force 5 spell to surpass the "extreme" OR threshold of 4+, which is not that easy to pull off. Still it's too easy, especially for those "natural materials" like, say, a whole forest..
I guess just adding a "volume of affected material" qualifier to the threshold would calm the spell down considerably. Like +1 to OR/threshold for every m³ of affected material beyond the first. That way a tree with a volume of 3 m³ would have a threshold of 3 while a barrel of crude oil would have a threshold of 1 still. Meanwhile the citymaster with a volume of, say, 4 cubic meters of metal/material would have a near impossible threshold of 7 to set alight, though its tires would be a simpler threshold of 2.

You're still thinking about ignite as a shroud of fire applied to an object. That's not what it is. It's kenetic agitation until the object itself is so excited that it begins to combust. By its very description, the material in question, is on fire. It's not a fire applied to a target, but a target which has gotten so hot, that it bursts into flame. The material is the source of the fire. I guess you could compare it to the mutant power of Gambit of the X-Men. The mage excites the object's molecular structure, like some kind of psychic microwave, up to the point that it catches fire and ignites nearby combustibles.

Also, a citymaster would need at least a force 5 spell to surpass the "extreme" OR threshold of 4+, which is not that easy to pull off. Still it's too easy, especially for those "natural materials" like, say, a whole forest..
I guess just adding a "volume of affected material" qualifier to the threshold would calm the spell down considerably. Like +1 to OR/threshold for every m³ of affected material beyond the first. That way a tree with a volume of 3 m³ would have a threshold of 3 while a barrel of crude oil would have a threshold of 1 still. Meanwhile the citymaster with a volume of, say, 4 cubic meters of metal/material would have a near impossible threshold of 7 to set alight, though its tires would be a simpler threshold of 2.