"Guns can't work in the dungeon"
Oh god, this shit again.
How does the dungeon know what a firearm is? When does the dungeon consider a weapon to be a firearm? It mentions "bullets," so we're talking about projectiles as well. Then I expect bows and crossbows, slings, etc. to not work as well, which I fucking guarantee you will be allowed because... reasons... And if a crossbow, which is mechanical, works there's no reason other mechanical things shouldn't work
I'm going to guess wind-related magic works and the idea of a dungeon being able to tell and differentiate between wind/air just being used and it being used to propel a projectile is even more asinine and ridiculous than firearms not working.
So we're just going to use advanced pneumatic air guns. Problem solved. Fuck your bullshit dungeon rules. Those dumb game-like mechanics will be broken on day one.
The thing that unnerves me most is you don't even need this dumbshit mechanic in your story. Just have the dungeons that do pop up in Japan, where they even have fucking sword control and are never going to allow civilians with firearms to go down into the dungeons.
I mean, America is another story, we're going to be running wild down in the dungeons with suppressed 300BLKs and NVGs just because we can and find it amusing/fun. Doesn't mean that has any bearing on Japan, which is if they do have civilians going in are going to be prohibited from using firearms, even likely swords, and would likely have more people willing to use just magic and melee weapons.
(Although for that gigachad stacked Japanese dude that goes in there and judo-throws the fuck out of monsters has my respect. That's just badass.)