The Tavern

If you want a 1h launcer, you can go slings, if you want a 2h one, you can go bows. If crossbows are meant to be weaker than bows, and to make a species weaker at ranged by giving it a better crossbow apt than a bow apt, then you can simply give it a bad bow apt instead. If they are there to make monsters with crossbows behave differently from monsters with bows, there's no need to make crossbows usable for the player.

Do crossbows have an objective in DCSS game design? Or do they just happen to be there?

Crossbows can get piercing?

But yeah, I think in general they're there for the same reasons as long blades and m&f. History, partly, and partly so there can be more difference between species, and partly so you can have a wider variety of artefacts in the game without them being useful to too many characters.

I'm still in the "remove all missiles" camp (a lonely campsite) but Damnation is a cool crossbow so

Crossbows hold the unique (among launchers) ability to use the same skill to use a 1H weapon or a 2H weapon. Bows are all 2H and Slings are all 1H unless you're a Small race, whereas crossbows come in both 1H and 2H varieties and use the same skill.

In 3-5 rune games at least, once you get to vaults you don't have to worry about ammo for crossbows, but arrows can get scarce if you don't have Snake.

Wait, what? Did they remove centaurs?

No, but I've found yaktaurs to be more reliably plentiful. I know, I have enough arrows if I only use them in important fights. But I find that too tedious and tend to just Tab.

They also have a much higher base damage for piercing AC and a higher skill cap for going super late game.

They're more accurate, for that matter. I don't have hard numbers for that, but the wiki says it (so it must be true, right?) and anecdotally, I can't ever hit a damn thing with a bow, sometimes hit things with slings, and generally have a >50% hit rate with crossbows when I have a reasonable skill level (comparable to the other two).

Take that with as much salt as you'd like.

Also no one's mentioned the main defining characteristic that is unique not only among ranged weapons, but all melee weapons as well: Crossbows have a min delay of 10 auts. That's quite significant, especially if a fight starts to go poorly. If things are going well and all your threats are still far away from you, shooting them at range isn't too risky even if it's slow.

The hard numbers for it are in the item descriptions...

Oh yeah, so they are. I had somehow managed to completely forget that accuracy rating was in weapon descriptions for a moment.

So, in practice, the difference is that you can use a crossbow somewhat more often because the ammo is more effective, but you want to use it when the monsters are further away, so that the long delay doesn't end up with you being hit?

How does delay interact with spellcasting enemies?

In the late game, longbows are simply better. They are faster then the big crossbows, and 7 vs 10 aut delay means +30% damage. A higher base damage does not compensate for that.

It is the early game where crossbows are good. Shortbows are total crap while hand crossbows are not.

Yes, actually, it does. While only part of the story, the 15 base damage for longbows Plus 42% (That is 10/7) for speed gives you 21.4 which is very slightly worse than the 22 base damage for triple crossbows.

A Longbow is very very close to a triple crossbow on average (presuming you have 27's in the relevant skills), the triple crossbow does slightly better if you have much better than average strength (the base damage scales better with strength), or are fighting things with very very high AC (like orbs of fire) the longbow (because if it's speed) scales slightly better with slaying bonuses.

Given a "middle" case like a +9 freezing version of both against a stone giant with 10 strength, and no slaying bonuses, they are nearly identical.

Do some Fsim's if you'd like to check for yourself.

It takes 194% as many skill points to take a triple crossbow to 10 delay as it does to take a longbow to 7 delay. According to the good old objstat tables, longbows are also more than 30 times more common than triple crossbows, though this part is mitigated later in the game by worshipping Trog or to a lesser extent Okawaru, which you probably are if you're doing a missile character.

My suggestion to make crossbows more unique is to give all crossbows innate penetration (and nerf the damage a bit). I know this was tried and reverted with large rocks in the past, but putting it on an entire weapon category should be a lot more interesting than putting it on one missile that only two species can use, because now there's an actual strategic choice: choose between bows' superior single-target damage or crossbows' penetration. It would also have an amusing interaction with the rare chaos crossbow and Damnation (which also needs a nerf).
If the flavour of crossbows having more penetration than longbows is too unbelievable, you can rename crossbows to heavy firearms, or magic rods or whatever.

This still leaves slings pretty redundant with bows. While having separate 1-handed and 2-handed launcher categories further enforces shields as a strategic decision that's painful to reverse (you don't train shields skill until you're late in the game or want spells, but you need to train launcher skill immediately), you could achieve that just as well by making the shield penalty for weapons more significant, making you train Shields earlier. It's also kind of hard to imagine loading a sling with one hand.

I think that your idea to give crossbows innate penetration is a great idea. An interesting difference. It might make that artefact longbow Piercer feel a bit wierd though. If I find an early buckler though and can make use of it, I tend to train the sheild skill to 4 very quickly.

This is definitely true, End game-level crossbows are both rarer and require much much more XP than longbows, I wasn't arguing that they didn't (or that crossbows compete *overall* with bows for effectiveness, particularly when looking at the part of the game where XP is a concern) Only that crossbow's ultimate damage capability isn't any worse than longbows (And in some cases is better)

I was thinking about ancient weapons with penetration, and the only ones I could think of that could hit more than one target with the same projectile were massive wall-mounted ballistae, that didn't work like crossbows and instead used rope torsion.

So I thought that it could be a more fantasy class of weapons. Some sort of gun built of precious stone that consumes the (mystical) energy of crystals to fire a penetrating ray of light. Something like "amethyst/ruby/diamond discharger" powered by "discharger crystals".