as effective as a standard strength firespark at 50 shots per turn.
I treat firearms like spells with material cost , but with physical damage types.
making decent quality ammo requires heavy tools , making ammo creation during travels a pain in the ass while mana simply regenerates, and sharpening a stick + putting some feathers on it can be made with minimal equipment.
using magic also means having multiple different guns at your disposal , may it be homing missiles , javelins (the spear) , javelins (the rocket), mortars , flamethrowers , or good old boolits.-all of this combined in your own body or a 3-6 kg staff as a catalyst. even if you are literaly disarmed , you can still learn to channel magic through your upper arm stumps and aim using your mind.
gunpowder & metal ammo on the other hand have a far higher potential reach and dont cause magical sickness once being used too often per day, and allows to instantly attack rather than prepare the spell for a second first. therefore , guns are the "magic" of dex and str builds, but having a somewhat decent variety of weapons can slow you down a lot.
another thing is , while magic directly improves with its user , you need a gun with bigger boolits if you want to use it against stronger targets , increasing recoil and ammo cost & weight on higher levels.