Most settings with magic involve some kind of restriction or cost for that magic, to prevent mages from simply using magic constantly to do anything. Sometimes it requires sacrifice, or mages have a limited mana pool that must be recharged by ritual or rest, or even just needing to clear one's mind and focus to cast properly, or it might take time for a person to pronounce the words to a spell. In some settings a spell can fizzle or worse, catastrophically misfire.

Presumably a gun would not have these same limitations, and using magic to empower or guide a material projectile would be easier or less costly than attacking with magic directly, since the magic is doing less. As long as the cost for using magic to mimic a gunshot is higher than just using the gun itself, there will be a reason for mages to carry guns.

You ask

how would mages find guns practical when they could already sling around fireballs and such?

Well, in our world we have guns, too, but sometimes people use blades or even steel wires for scopes where the guns could still be used? Why?

Detectability and circumstances

There are tactical situation where using a certain type of weapons is not advisable. If you are a hit-man and want to kill your target intruding a highly guarded location, the loud bang of a weapon firing might make the job more difficult, so a blade would be preferable.

Same goes for magic: if there are ways to spot the usage of magic, using guns might be a good way to pass unnoticed, or even fool the opponent into believing something false (A is using guns, thus no risk of fireballs), which can be a tactical advantage.

Why do modern soldiers still use guns, when they have rocket launchers available? This should answer your question regarding guns versus fireballs.

Even if the guns aren't enchanted in any way, there are still many situations where they are superior to magic spells.

1. In a self-defense scenario, or if the target is very close, you might hurt yourself (or innocent bystanders) too with your spell. Be it fire, or electricity, or ice, or anything else, if you are too close, you could be also affected.




2. Firearms are faster. In most settings, magic needs fancy hand motions and the uttering of magic words. In the time one says the first syllable of a spell, someone with a gun can draw it from the holster and shoot. "Avada Ke-" BANG-BANG-BANG!




3. If your magic uses wands, at longer distances guns are way more accurate. Wands don't have sights, and even if they did, guns have better grips so you can hold them in a way the sights are aligned with your eyes. Unless you make your wands with gun-style grips and put sights on them... but if casting spells needs fancy waving motions with your wand, then you have a problem.




4. If magic exhausts you, or otherwise uses up a limited supply of "mana", you might need something to defend yourself with in the case you run out of magic. For example, in Vancian Magic (used in most roleplaying settings), even a very skilled wizard can cast at most a few dozen spells per day. You can carry much more bullets than that, and you don't need a full night's rest to pick up more bullets.

Perhaps in your world magic is easy to see coming and not terribly hard to protect against, if you're trained to do so.

Bullets are a lot harder to protect against because they're very fast and hard to see and physically quite powerful. Some magical worlds allow people with the proper training to put up bulletproof shields, but this may not be possible (or easy) in your world.

Fire is messy, bullets are clean(er).

While a bullet can go astray (if one doesn't use magic to guide it), it won't cause harm to anything unless it hits it directly. Secondary effects are rare (like a bullet hits something hard enough to make it fall down). Generally, your bullet either hits your target or it doesn't.

Fire though...fire can light things on fire. If you miss with your fireball, you can cause a lot of destruction. The curtains catch on fire and the house goes up in flame. The fireball lands in a vacant lot that needs to be weed whacked and, an hour later, the neighborhood is on fire. Maybe your magic can stop it, maybe you're too busy fighting the person you just failed to kill.

Even if your fireball hits its target, you can cause a lot of destruction. The purpose of a fireball is to catch something on fire. It's hard to limit its scope. Then you have ash and embers and it's a big expensive and dangerous mess.

Who needs that hassle? Just get a nice pistol or shotgun and go take care of business.

A justification I haven't seen mentioned yet, but which I have seen used by Harry Dresden of The Dresden Files fame:

Magic is dangerous to use

In the Dresdenverse, magic is powerful, but also dangerous. You need decades of training before you can use it, and you need special incantations you don't know the meaning of to properly channel it without risking damaging your own body. Magic also is hard to control and can easily backfire. You need bulky tools to make effective use of it, like a staff or a wand or a blasting rod, tools that you can't always use because of space restrictions. It has a lot of unwanted effects on anything which involves electricity, so you can't use it in a place with a lot of electronics or when electronics are needed for basic survival needs. It is also is either massive overkill or woefully underpowered for most enemies, depending on the enemy type. Many creatures are immune, others have heavy resistance, and others cause side effects when attacked with magic.

Because of that, Harry carries around a gun and knows how to use it.

Power, time, effort & detection/defense.

Power: Whatever powers your spells, that might be limited somehow. Certain number of spells per day, certain amount of magical energy and so on. Basically, a gun can be a sort of "hold-out blaster." Note that cost doesn't have to be higher to use magic, just of a different "denomination."

Time: Executing a spell may take a little time--hand gestures, magic words.

Effort: Spells may take concentration.

Detection/defense: If there's a lot of mage vs. mage espionage and such, a mundane piece of killing might actually be better. It doesn't show as magic, and if defenses are built more against magical attacks, a gun might be best.

BONUS: Magic can be unpredictable + there may be anti-magic or wild magic geographic areas that mean a gun is a better idea.

& as Scott said to his dad in Austin Powers "Why don't we just shoot him now?" https://www.youtube.com/watch?v=5xAMYHJYesM Same point Cyn made in another post, why worry about an unnessarily exotic death when you can just shoot them.

It's actually pretty telling that the question "Why don't wizards just use guns?" has come up in the past on these boards. You're asking sort of the opposite question (though related) of why they would bother to. It actually doesn't make sense that they don't...Most of Harry Potter's dilemmas could probably been solved by sneaking up on Voldie and shooting him in the head.

Look at your traditional eight schools.

If you're a specialist in "protecting stuff" or "knowing all the stuff", you probably don't know many offensive spells. If you're an enchanter or illusionist, you might have spells that can hide you from an attacker, or trick an enemy into walking into traffic, but these depend heavily on the situation.

And fireballs cause a lot of collateral damage.

Brian McClellan in his "Powder Mage Trilogy", does exactly this.

His powder-mages derive their power from snorting/inhaling the gunpowder, it enhances their ability's and perception and gives them an edge battle.

In use they use it to ignite gunpowder from afar, shoot multiple bullets (Not modern cartridges, packed gunpowder and lead balls) from the same gun, enhance their range, and guide bullets to their intended target. They can also use it to locate their enemy by sensing the gunpowder they carry (If they have any).

As for where it could fit in, Magic such as fireballs may only be able to be launched so far; they may also drain the energy of the mage whilst it is in effect. It depend on what restrictions you have used for your mages.

EDIT: Don't forget that Firearms are primarily a tool, rather than a weapon. As such it could be used in the same manner as a staff or focus. A smart mage specializing in Evocation could create a small explosion behind the projectile, eliminating the need for gun powder. He could imbue the projectiles with Necromancy, the corpse created buy the projectile rising to attack its comrades whilst the mage spectates at a distance, smiling gleefully at the chaos he has created.

Maybe areas have an anti-magic shield, where someone can't cast a spell. But they could shoot a gun from outside the region (if the guns or bullets needs to be magically enhanced) or inside the shield if it's a non-magic gun. Or another varient is they can't cast a spell in a place without setting off a magic alarm but they could cast a spell yesterday when in their own home and "store" it in the bullet. Then when they fire then gun today they technically haven't cast a spell so won't be detected.

Mana, juice, energy or whatever you call it

Mana is the power source of a mage. As they exert their will on the nature, they deplete their mana source. Casting larger projectiles and hurling them forwards such as fireball will deplete their power faster. They better use their energy to imbue an already semi-deadly weapon to conserve their mana for the rest of the day.

Additional boost

A bullet is quite a weapon, it has long range, it is quite deadly. Maybe the fireball of your mage is also as useful. But the combination is far more deadlier. Due to the construction imperfections, early firearms was not very accurate. Guided bullets will be as deadly as someone can get. Armor penetration is quite useful too.

Guns could enhance the ability to spellcast (especially if you have magitech) in many ways. Don't just think about enchanting the gun to be a better gun, think about the gun enhancing the magic.

You could store spells or potions in bullets that take a long time to prepare. The gun could then deliver the magic/potion to the target, if the bullet is faster then the spell would be (or if the spell needs to touch the enemy).

I think it would depend on how easily magic is learned. In many settings, magic is a highly specialised skill that not every magician is highly gifted at. You say:

anyone can become a magician if they choose to dedicate enough study and practice to it.

But one could say the same for almost any subject like science or mathematics, that doesn't make all of us incredibly skilled at these things. Some people devote their entire lives to these practices and still won't have a comprehensive knowledge of these areas.

Comparatively, weapons may take significantly less training to use and wield. This means that any combat wizard, regardless of specialty or training, could at the very least have enough proficiency in firearms to defend themselves if cornered or surprised (in the same way that longbowmen would still carry daggers because there may be occasions where their primary weapon may be more unwieldy).

You haven't really told us HOW magic works in your world. Does casting a spell take time? does it take resources(physical or mental)? How does enchanting work? Do spell auto hit or do they require precise aiming?
here are a few scenarios:
magic doesn't require resources and is instant: no real use for guns unless the bullets are enchanted with something the mage can't himself cast

magic doesn't require resources and is NOT instant: guns obviously have great use here, they let you react much more quickly

magic requires resources or the number of different spell you can memorize in a day is limited: enchanted guns are great here for several reasons, they help you avoid having to carry a large amount of ingredients like pearls a perfect feather, metal shavings, blood and other goo. If the the number of spells you have are limited than pre-enchanting a vast amount of ammunition is a fantastic force multiplier, especially if you have automatic weapons, can you imagine even completely non harmful bullets enchanted with even a small splash effect in a machine gun, now imagine that with fireballs....?

Aiming is also an issue you should consider, rifles are relatively easy to aim, you can get all sort of attachments that make life easier like scopes and range finders etc... how do mages aim in your setting? just point at a spot? does he have to form some sort of line? just focus on the spot? if its more complicated than vaguely pointing, then guns are probably a good choice, especially for longer ranges

also consider them a status symbol, an ornate pistol to symbolize that the mage is rich enough to have others enchant his bullets for him so he doesn't have to memorize how to cast fireball 5 times every morning

A gun is basically a multipurpose rechargeable wand with near instant activation, the pistol/rifle shape is really just cosmetic/thematic

There are lots of reasons.

Energy: You don't have enough energy to imbue your bullet with fire and shoot it simultaneously, so you let the gun do the shooting.

Skill: You're still getting the hang of magic, and it's a dangerous place, OK?

Reliability: You come home from a tiring day of work shooting lightning at batteries at the magitech power plant, and get mugged on the street while your power is drained.

Convenience: Magic is hard, guns are easy. All I need is to shoot a bullet at something.

Simplicity: Oh, crap, I'm being attacked! Ok, what was the 'shoot projectile' spell again? Did it start with 'Alacanamen' or 'Sephicus?' Do I raise my right index finger or my left pinky? Screw it, out comes the pistol.

Specialization: Dammit, Jim, I'm a healer, not a battle mage!

Time: Point-and-shoot is faster than getting out the grimoire.

One-size-fits-all: Most soldiers in the army aren't expert mages, so we just give guns to everyone.

Assuming using magic takes some time in form of moving hands, speaking some words etc. there would be one obvious reason to use guns instead of (or rather as an alternative to) magic:

Bullets are fast

Much faster than most of the magic you could use. In the time it takes you to even swing your arm to unleash an attack, someone with a gun could have shoot you a couple of times (Or, assuming a sniper rifle, even killed you with one shot). When you cannot keep up with the speed of bullets, even the most superior attacking/defending/repelling spell is utterly useless, since you get no time to cast it.