So we get the rules for crossbows in 5th Edition on p. 18 of Covenants...except, of course, what ability governs their use. What say you? Bows? Or a separate Crossbow ability?
Pax,
Patrick
I went with a separate skill. Some people will argue that it should just use the Bows skill, but I feel Bows and Crossbows are about as different as Single Weapons and Great Weapons.
What he said.
Otoh, if you're the SG, you tell us. 8)
I was just thinking the same myself today. I also worry that crossbow is supposed to be easy to learn, longbow hard but in Ars they would be equally easy.
(in theory, that's part of where the Attack modifiers come from)
Was actually surprised of lack of crossbow rules in 5th edition since their use was spread in 13th sentury Europe. Allthough theyre use agains other christians where banned by the church, but on the other hand so was tournaments.
I think using of two separate skills is closer to reality, I've tried both IRL (archery is one of my hobbies).
I wouldn't bother with a separate skill from Bows. There are enough XP sinks already. Adding a new skill that only correspond to a unique weapon doesn't really ring with "easy to learn". If you wanted them to really be easy to use, you'd put crossbows on the Brawl skill, which is available to just about anyone.
Putting it with brawl of bows seem not right.
You could make another skill for the crossbow but consider anyone learning it as if the character have automatically an affinity with it to simulate it's simplicity to learn.
I'd rather just give the crossbow a higher attack bonus. It would achieve a similar result and it wouldn't be as fiddly.
As someone mentioned, there are already a good number of skills to sink xp into. I'd chuck it in with bows if you wanted "bowmen" to be good with all "bows" or, and I do like the idea, stick them under brawl to represent just about any thug/lout/soldier being able to handle them to some degree.
I'm quite tempted to do the second myself.
Touché 
Both rules might make some rough sense in 13th century campaigns run in specific locations - like the Italy of the comuni - and not going to leave them. But if you instead consider rules which work all over Mythic Europe and for a few centuries, giving every brawler or bowman also Crossbow skill will lead to an ahistorically wide distribution of it.
Given the +5 Init and Atk bonus of the Covenants Crossbow, even after just one season's drill (practice with feedback) in a separate Crossbow skill your average healthy covenfolk will be as proficient with it as after six such seasons' drill with the short bow - so this already accounts nicely for the relative ease of learning the crossbow, without making every prize-wrestler into a sharpshooter as well.
Kind regards,
Berengar
This is a great solution, given that you take no attack penalties (but 3 extra botch dice) with an unfamiliar weapon this would still make the crossbow quite useful for the untrained.