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?
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.
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...
- Is inexpensive. It's just a well-decorated tube (OK, it's more than that, but you get my point),
- Is trivial to maintain,
- That does not depend on plentiful dry powder, patches, etc., and...
- That is much simpler to load (and you can do it in a thunderous downpour!).
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.