As someone who has made many such firearm suggestions, let me put my two cents on this.
Objectively: Mojang isn't going to add firearms as a rule. This is rather illogical, seeing the plethora of violent content Minecraft already includes, but neither is it something that they are really going to shift on.
Objectively: They don't read these suggestion threads; this entire forum section is just brainstorming by the community for the community. There are infinitely more fitting and polished suggestions- Ant Dungeons, Desert Temple Golems, etc.- that are full of assets and beautiful presentation, and they have vanished into the sands of time without a glance from the company. So it's quite pointless to say "Mojang won't add it" in this context- the best it can expect is a passing modder to take an interest and spin it into a playable thing. (Speaking of that, allow me to shamelessly plug our modding project doing exactly this to forum suggestions and more...)
Now: does a gun fit into Minecraft? Short answer: yes, surprisingly.
We can divide "fitting" into two categories: theme and mechanics. Theme being how well it meshes with the look and feel of the game (driving a Mustang around might be fun, but it'd stick out like a sore thumb with the rest!). Mechanics should be self explanatory- how it affects the balance of the game and how it behaves. A magic wand might make sense thematically, but if you can make it 5 minutes into a new world and it insta-kills every mob in the game without using any ammo, it's also unfitting.
Thematically, Minecraft is a schizoid mix of pop-culture fantasy of all stripes, and one of the most unique ones around. There are your usual trappings- swords and plate armors, bows and crossbows, zombies and skeletons- but also a variety of aesthetics and blocks alike incongruent with this. Sunlight sensors and chocolate chip cookies are inventions of the 20th century, TNT hails from the year 1863, so on and so forth. (The latter can't even be excused as a small flavor item, or magical- it's a bunch of big red sticks straight out of a Western!)
Guns, in their earliest recognizable forms, originate in China in the late 1200s. By the mid-14th century they had reached Europe, and coexisted alongside plate armor, bows, and swords right up to 1700. There's a number of medieval firearms available to choose from- but all are basically similar in being iron tubes sealed up at one end, with some kind of wooden handle to hold with, and either a simple touch-hole or spring-loaded ignition mechanism. This is the sort of thing Steve can conceivably build: he has every single item he needs, in fact, already in game.
Mechanically speaking, there's no real issue towards balancing a firearm. The existing ranged weapons each fulfill a role:
- Bows are accurate and strong, with enchantments offering great power arrow-per-arrow, but relatively slow shooting.
- Crossbows are effective crowd-control weapons with enchantments and/or rockets, and enchanted the fastest of all.
- Tridents are powerful at short range, and as melee weapons too- plus are the only credible underwater ranged weapon.
Guns would lack the precision and range of bows, or the speed and crowd-control of crossbows- but would reign as the kings of single shot firepower, particularly against armor. Here's a breakdown of the basic mechanics:
- Aiming and firing would be tied to RMB, same as the Bow. Accuracy scales with aiming time- it would be able to hit reasonably well within 30 blocks after several seconds of sighting in, but just tapping RMB means the shot'll go wild anywhere short of point blank.
- The actual shot produces some smoke particles and light. Bullets would travel twice as fast as arrows, on a somewhat flatter trajectory, and leave thin wisps of smoke in flight marking their path.
- On impact: deals 5 hearts (10 HP) damage, same as a fully charged Bow. However, there is one special effect: 50% of enemy armor is ignored. If the bullet hits a shield, it'll penetrate right through- but then lose its armor-piercing effect, being the same as an arrow from there on.
- Instead of an active reload the musket would use a cooldown similar to Ender Pearls- this being to prevent spamming bullets down a hotbar like you would Crossbows. The cooldown would last 6 seconds.
- Enchantments would center around the effect of the shot:
- Piercing reenables the armor-piercing effect after passing through a shield, and otherwise works like it does on a Crossbow;
- Sundering is mutually exclusive with Piercing, and adds 10% more ignored enemy armor per tier up to IV (for a maximum of 90%).
- Shellcracker- after being hit, a target would for 4 seconds suffer 10% reduced armor effectiveness versus all attacks, adding +10% per tier up to IV (for a maximum of 40%), independent of the bullet damage.
- Mending, Unbreaking, and Curse of Vanishing would exist as usual.