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So, in this fantasy preindustrial world almost everyone can cast small spells like spark within a one meter radius from their mouth.
Taking that into account, igniting mechanisms like matchlocks or flintlocks might seem redundant.
What would firearms look like and function in this world? And what would be the simplest firearm mechanism possible?
Huge impact
Given that the OP specifies that the "Spark spell" can ignite powder through a metal barrier, firearms are suddenly much more effective at each stage of development. The increased effectiveness will make them more desirable, which will push the impetus to design improvements. Let's look at what makes them better.
The combination of the above points will make firearms far more effective from the moment they are first introduced, which will in turn make it a far better investment to work on developing improvements such as sights, rifled barrels, more consistent gunpowder etc. Also note that the Spark spell would have a lesser impact on improving both land and naval artillery, as the need to cool down the barrel of a cannon in between shots would make the increase in rate of fire negligible.
Possibly unintended consequences
The ability to Spark through metal and presumably other substances may have issues if control is difficult to achieve and/or suicidal opponents are encountered. The "one metre from the mouth" limit prevents deliberate widespread battlefield "jamming", but it should be noted that every person with a pre-industrial rifle will also have a powder horn or the equivalent, which is the equivalent of a grenade sitting against their body. A recruit with poor control who is shoulder-to-shoulder with his fellow soldiers could conceivably detonate not only his own powder horn but those of the soldiers to either side in one fatal mistake. Similarly, an apparently-dead-but-actually-still-dying enemy on the losing side could, with his last breath, detonate the powder horns of one or two soldiers on the winning side as they advance across his body.
I don't believe anything would be different in terms of advancement
It might be uber cool to cast Spark! to ignite your firearm, but it has several limitations — and one is, IMO, a doozy.
First, the use of magic with a firearm is splitting concentration. From the perspective of real firearm use (which we need to consider if we're going to answer the question, "how would they develop if...") you're focused on your target and your focused on your weapon. Now you're also focused on the magic.
It's slow. The time required to invoke Spark! will always be slower than the time required to pull a trigger. Even if we go back to the earliest guns. Anyone with a trigger-activated gun will have an advantage over anyone using a magic-activated gun. It won't take long for everyone to have a triggered weapon.
Here's the doozy. The incantation can be heard. Whether your hunting or a soldier in the military, the sound of the incantation will always precede the ignition of the powder. Generalizing, one can only hear the report of a rifle before dying when shot from a great distance. In most cases, you either never hear the shot or, more to the point, you can't react to it. Enter the incantation, where every target not made of bottles, cans, or wood will hear you speaking before the bullet is flying and have a chance to react. That gives a good advantage to the prey when hunting and a substantial advantage to an enemy soldier who's opted for the much more stealthy triggered weapon.
Now... if the incantation for Spark! was nothing more than an unbelievably quick and an equally unbelievably soft click of your tongue.... Can you create the magical version of a hot-key?
Conclusion
I can believe a series of magic-activated firearms would exist, but somewhat as a novelty. The aristocracy would love them. They'd be seen at gun ranges.
But I believe that the advantages of a trigger-activated firearm would make them preferred by the military, for hunting, and for personal security. That's 99% of the firearm market.
Alternative
From a certain point of view, an incantation like Spark! would represent the least a magician could do or, said another way, would represent the smallest change in the operation of a firearm. Is that useful?
What makes a firearm deadly is its ability to accurately direct the projectile. If you don't have something like Lightning Bolt! that "automatically" hits its target, then it makes more sense to replace both the trigger and the powder. Rather than casting Spark!, you want to cast Bang! All three issues mentioned above technically still exist, but now what you have is something...
I'd recommend that you modify a world rule to minimize the sound of the incantation, but...
All those advantages weigh well enough against the disadvantages that my suspension of disbelief feels like it's happily eating pizza and drinking a cold soda.
Blowpipes/Blowguns. They've been around for quite some time here on earth as well, and it wouldn't be too far of a stretch to adapt them into a form of firearm if your citizens are able to cast a spark from their mouths. I think the biggest issues might be recoil, and keeping any expanding gasses from blowing back out of the end that faces their mouth. Though if they can cast the spell in a one-meter radius away from their bodies, they might be able to get around that issue! Or if they engineer some sort of valve that shuts off as soon as whatever charge is inside the pipe ignites, which wouldn't be too complicated. They also might need to make them out of sturdier materials as well though.
In close quarters, can one put a spark in your eye? Heart? Head?
Is that survivable?
Might be an interesting addition to the second act of the classic battles, when two sides have exchanged a few volleys of balls and the muskets became clubs with knives on the end (bayonets) and so the distance to opponent is close enough for the one-meter range.
Otherwise, in some hide-and-seek thrills, nowadays characters often toss a bit of debris to make a sound which makes the seeker momentarily look elsewhere (and the character has a better chance to ambush or run away). Can a spark be used like that (if the delay and spell spoken volume is negligible)?
Conversely, can a delayed spark be set? e.g. you run through a corridor, hide in a random room and make a "proximity spark" in advance (less limit on the spoken voice giving away your position) that would go off once someone comes through the door, so then you make your jump on the distracted intruder...
