There definitely shouldn't be a bonus for being on horseback - and longbows shouldn't be capable of being fired from horseback at all, and all but the lightest crossbows shouldn't be reloadable, since even medium-strength pull crossbows had a stirrup and required foot power to reload, which is basically impossible to do reliably on a horse. Simply being able to fire the shortbow from horseback at all is the only real bonus you need.
Further, longbows are just plain bulky - usually at least as tall as the person meant to be firing it, if not taller, and as such, it should be a serious encumberance to an adventurer trying to tramp around swimming in rivers or sewers, hiking up and down mountains, etc.
Crossbows can indeed be fired from horseback. While stirrup, belt hook, and windlass operated crossbows require the operator to remain on foot, the lever operated cho ko nu repeating crossbow and ratchet and crannequin operated crossbows do no require this. Mounted crossbowmen were used for a period of time in the middle ages/renaissance as they posessed significant mobility combined with firepower greater than that of the longbow.
For its part, the longbow was used in mounted combat in the hundred years war, although mostly in skirmishes and raids, from what I understand, and as a method of transporting archers to more advantageous positions for firing (at which point they would disembark) Longbowmen and crossbowmen were most often used on foot during combat, but this does not mean that mounted longbowmen and crossbowmen did not exist, mounted crossbowmen in particular seem to have been rather prevalent during later periods when armor was well developed but before firearms had developed to the point of outstripping crossbows.
IMO there needs to be an overhaul of how armor/clothes and weapons work in DF. Iron bows are just silly, and there definately needs to be soem way to differentiate ranged weapons on criteria such as range, accuracy, and reload time. Right now the only thing that makes them different is the skills they use, the damage their ammo does, and how much useless they are as melee weapons.
Crossbow firing sequence should be:
1) Draw string Dwarf has to remain still when doing this (cannot walk/run/ride horse) This step will take a long time
2) Load bolt (this makes the crossbow fully ready to fire, the marksdwarf can fire at any time after this action is completed) takes as much time as knocking an arrow
3) Fire arrow (takes as long as it does in vanilla)
Downsides would be having to unload and release the string before storage, and the long loading time. When item degradation is implemented, they would likely be rather suseptable to water damage. High power, high accruacy, long range, low rate of fire. requires both metal/horn/wood and wood to make, strength of bow dependant on metal/horn/wood type.
Bow firing sequence:
1) Knock arrow (Takes a short amount of time, as long as loading a crossbow bolt.)
2) Draw (takes a short amount of time, renders bow 'ready to fire' similar to loading, but can only held for a certain amount of time before the archer must release, dependant on strength. Low strength/large draw bows require archer to remain stationary in this state)
3) Fire arrow (takes as long as it does in vanilla)
Downsides would be less accuracy, range and power compared to crossbow. Archer cannto remain in 'ready to fire' state indefinately. Easier to make and maintain. Only wood or horn(for compound bows) can be used. medium power, medium accuracy, medium range, high rate of fire.
Blowgun firing sequence:
1) Load blowgun (takes a short amount of time, makes blowgun 'ready to fire)
2) Fire blowgun (takes as much time as it does currently)
Downsides would be poor range, power, accuracy. Very easy to make. Can be made out of many materials. Low power, low accuracy, low range, very high rate of fire.
Thoughts?