The longest range sniper kill was 3.5k meters away.
the average military sniper kill is 600-1000 meters away
An average gunman would kill an average magician easily don't compare the bottom of one to the top of another.
The fire suppression spell has to be casted on the area where the gunman is / on the gunman.
Assuming the fire suppression spell is the control flames spell from the DnD 5th edition it only has a range of 60 feet or 18.29 meters this can be doubled up to 120 feet or 36.58 meters but this is if you expend 1 sorcery point and already have points in metamagic ( according to DnD 5th edition)
Also a Magic fire arm can get around the control flames spell by just making the initial spark a magical spark therefore mitigating the control fire spell in its entirety because according to Xanathar's Guide to Everything for DnD 5th edition it can only effect nonmagical flames.
And for a normal firearm yes it is just nonmagical piercing damage but does the average enemy die to normal nonmagical dmg? yes. A goblin would be clapped by a gun a big ogre or ork can be put down by a gun. Even if the enemy isn't easily effected by non-magic piercing damage a gun would still be able to hinder the enemy far better than a non magical sword or bow because of the kinetic energy being imparted from the bullet to the enemy.
So yes a gun especially a magic gun would be a significate advantage to anyone in a fantasy world.
If you bring up stuff like magical dragons I'll bring up magical anti-armor. non magical dragons? Normal Anti-armor. The firearm tech tree is just too good.
Also if you're willing to accept a mythical dragon being defeated by a knight or a hero party consisting of bows and melee weapons why is it so hard to imagine a mythical dragon dying to a gun or a bigger gun?
The longest & average range
magic military & sniper kills are, of course, unknown. You therefore can't make definitive claims about the combat capacity of an average gunman in the real world vs. imaginary fantasy bullshit in general.
If you wanted to bring real world guns and gun data into D&D, you'd first have to give some serious - and I mean
serious - thought to what might be possible given
centuries of focused, GP-be-damned development of hardcore, super-lethal, military grade magic at the national level between ultra-powerful warring states. D&D has never done that, but the killing potential is obviously vast.
I do agree that a fire suppression spell would have to be successfully targeted to be useful.
Control Flames is a cantrip. All D&D spells are modifiable/levelable, so one should assume that the 1st, 3,rd, and 5th level equivalents (etc.) are
much more powerful, with greater range and area of effect. Considering the civic utility of such a spell in emergency situations and its combat utility whether or not guns are involved, I would assume that it would be well-developed and widely practiced.
I don't deny that guns are more powerful than bows and swords. They definitely are. But a 1st level character armed with one revolver and a very limited supply of bullets (i.e. the sort of firearm that might be appropriately available at 1st level) would not have an incredible advantage over other characters. Of course magic guns would be more powerful still, though that's no more true of guns than any other type of weapon.
As for enemies of various types, it would depend on the attacker's level-dependent chance to hit the enemy's armor class - along with whatever other adjustments might be involved - just as with any other weapon. At low level, more powerful enemies are often extremely difficult to hit, no matter what sort of weapon one is using.
I would agree that one can break any game by allowing low-level characters access to magic and/or technology that's vastly more powerful than what
should be available to them. A fully-automatic assault rifle, for instance, is much less powerful than a staff that can endlessly shoot 10d6 Magic Missiles at a rate of 100 per second. But neither should ever be available to lower-level characters (except perhaps briefly, to accomplish a specific mission or just have fun).