Good suggestion. Thanks!
When I saw this, I was doubtful and honestly thought, "no way." Most people think that when they see a gun suggestion, because most gun suggestions are either "lol add musket that do 15 hearts pls" or "lol add revolver".
You bring up a good point with TnT. However, still, Minecraft has always had an element of medieval fantasy, like the middle ages if they had magic. Swords and bows are obviously medieval. Potions and tridents are much more magical. Guns, though? I'm not entirely convinced. Still, it helps that you're talking about muskets and not AK-47s. The first arquebuses- the matchlock guns upon which the Musket's based- came around in 1450. The full set of Gothic heavy plate- I.E. the archetypical "knightly armor"- fully developed by 1535, mostly in response to musket fire. So really, muskets are more "medieval" than full Iron Armor if anything.
Ignoring that, let's look at game balance. I initially thought this would be OP and unskilled. A weapon with a very fast projectile and high damage? Might as well add a sniper rifle into the game. But the inaccuracy and reloading time you mentioned are key. A player can't just turn around and shoot to deal a million points of damage, and with the slow reload, it certainly fulfills a different niche than the bow. But because it's so slow, it would require more of a camper playstyle which, in my opinion, would be pretty boring and encourage the wrong type of gaming. With muskets, you'd have players hiding in a corner with an aimed gun waiting for someone to come so they can fire and immediately switch to their swords to finish them off. There's no such thing as "wrong type of gaming" unless it's blatantly unfair to other types. Muskets, strategically, can turn the tide of battle, but only when they're used well, such as the aforementioned corner strategy. That can be easily counteracted by using a Shield- causing the Musket to lose its armor piercing power once it penetrates, or simply neutralizing a player camping with Buckshot. The Musket player then has to commit to melee or run, since he has no time to craft some new ammo into the gun (hence why I put this system, instead of RMB-holding). Since he invested more resources into his gun than his armor, he likely will not win the melee against someone who went full armor + sword. He could instead win by flanking, sniping, or guerilla warfare, these roles being things the Musket is good at.
How do you plan on making its aiming look like? Like other gun games (quick zoom in when aiming, quick zoom out when un-aimed)? Will there be a scope you can aim through, or just a sight to look down? With extra damage, I assume there'll be extra knockback? What about knockback on the user themself too? Just like the Bow- screen zooms a bit as you hold down RMB. I like to keep things simple. Extra knockback seems good, so I will add that.
edit: ooh it's an Engie main >promptly creates Minisentry suggestion
Responses in bold.