Always good to leap into the fray very late in the process... (and this is going to be LONG)
First comment, remember that this is a game, not a sim. While many of us would like it to be one (at times, me too!), it is really about having different tactical options and using them as best as they can be used.
Summary of suggestions:
Shortbow 0 AP, -2 precision range short
Longbow 4-6 AP, -2 precision range longer
Crossbow 10-12 AP, 0 precision longer yet
Arbalest 14-18 AP, 0 precision longest
Slings (cheap) same as before
Slings (elite) 10, maybe 4 AP, -3 precision, between short and longbow range
Blowguns - put back at original, up the poison (curare/paralysis?)
New- Tien Chi'n repeating xbow -2 AP, 0 precision, 20 range, mild poison.
rate of fire: 1 or 1.5/1 (fast reload, easy cocking)
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Important notes: Longbowmen and the Slingers (elite) are elite and should
use the mechanism of having a high resource cost - they should cost as
much as a knight. Not because their equipment is expensive but because their training is expensive! There should only be a few available per turn. The other troops are militia or conscript quality.
Details (or why I'm shoot my mouth off (and maybe foot, too)):
AP - actually, any of the really pointy toys should be AP. Afterall, "AP" is nothing more than a mechanic to describe the physics of taking the force of the weapon and applying it to a very small area. Spear wpns in a charge or vs. a charge should be that way. Picks and the like are AP. All of the bow weapons are AP. Now, modelling that in the game correctly is going to be amusing.
Bows: actually, bows are AP only at close range and after that, they are going too slow to really pierce the heavier armors. However, the arrows inflict rather nasty wounds once they penetrate due to the instability of the flight path causing the wound to be ... complicated. The game can't support this level of modelling, iiuc. Also of note, different arrows were used for different targets. There were bodkin points for heavy armors, broadleaf points for no armor or v. light armors, and intermediate designs for other cases. This also complicates the game modelling problem.
XBows: they ARE long range, they ARE AP at all times. The xbow outranged the longbow. The quarrel is aerodynamically more efficient than the arrow at all points in the flight. IIRC, there was pretty much only one type of point for the xbow. This implies that it didn't really matter much what was used, it would hurt. (and I'm pretty sure the medievals checked this out - there are too many other examples of design improvements in weapons for that to have skipped.)
However, the biggest problem I see is that the troop recruitment methods are broken for distinguishing between these three troop types. Of the 3 - shortbow archers (sba), longbowmen (lbm), and xbowmwen (xbm), the lbm took a hideously long time to train. Recall that the ones used at Agincourt et al were the best of the best, sir! As such, they were HIGH morale, HIGH precision, and few in number. But, there is no mechanism in the game to model this. The sba troops were often levies, or issued bows on short notice, and the bow is a difficult weapon to master. They were lower morale, low precision, and expendable. The xbm were usually similar to the sba except that the xbow is v. easy to learn. So they should be low morale, high precision, and cheap in gold but less so resources. In game terms, one should only be able to recruit a few lbm per turn, while easily able to get many more sba or xbm. The only way I can see to model this is to up the resource cost beyond what the actual material costs are. Note, the national xbm (Marignon, for ex.) probably should be higher morale, better trained, etc. - they're not rabble given the death-dealer.
Another example of how "elite" the lbm were: they could pull a 150-200 lbs bow. This has shown up as distortions in their skeletal structure. The typical shortbow used for war is ~60 lbs. That takes loads of training. These guys were serious about bows. After Agincourt (iirc), the French tried to field their own lbm but failed. They didn't have the infrastructure designed to turn out vast numbers of archers that could then be culled down to those few that were superlative.
I would suggest going with AP 0 for short bows with a precision of -2/-3, AP 4-6 for the longbows with a prec. -2/-3 but the lbm are more highly trained!, and the xbows getting AP 10 or 12 and a prec of 0. Plate was pretty much only good for keeping the quarrel from coming out the back of the armor. The other projectiles operate under pretty much the same physics but just have lesser force behind them.
Slings: they are actually MUCH more dangerous than DomII models them. They are longer range than shortbows. They were noted for causing spalling to plate armors. For flexible armors, they were quite nasty as well. Note, the Rhodian and Balaeric slingers were much like the lbm - they were highly trained units and much sought after. Once the supply was wiped out, they pretty much disappeared from the battlefield. Professional slingers did NOT use random rocks found on the battlefield but instead used either cast lead or ceramic bullets - including incriptions and taunts cast in. If one wanted to mod "historical" slingers into the game, I would propose a range between shortbows and longbows, 10 pts damage, maybe 4 AP, precision of -3, but again, the troops are highly trained so the effective precision should be much higher. Gold cost would be moderate but the resource cost should be high - there are not that many of them.
------ of note: once the numbers of these professional slingers began to fall, the Romans opted to train new ones to use one swing around before launching their missiles. Previous accounts put the number at 3. Prowling the various sites that discuss slinging in modern times, folks claim to not be able to gain any advantage from doing more than once through the arc. I would put forth though that if the ancient slingers did 3, they meant to do 3 and that 3 gave them some advantage. Note that they were trained from a very young age to sling, unlike these modern blokes. The reason for the dropping of the number of swings by the Romans, as I understand it, was to make up in volume what they had lost in accuracy from using troops of poorer training.
Last bit: what of adding the repeating xbow for Tien Chi'n? It would be about as dangerous as the shortbow for impact, shorter range, maybe higher rate of fire, and with a mild poison? Troops were conscripts - point, shoot, run if out of ammo
Just to be complete: Blowguns are probably silly in the context of a sim, but fun as a game mechanic. No army ever fielded them. Why? They suck as weapons. The darts have horrible aerodynamics and the muscles used to propel them are rather weak. The physics of the blowgun are ugly for power projection. Practically, they are extremely close range and mostly used to shot vertically since they have huge, arcing flights. Personally? Leave the blowguns as is. Maybe make the poison more deadly? Mimic curare (paralysis)?
Sources: I'm doing this from memory with the following books and articles being source material. If called for, I can try to dredge through and find out why I wrote a particular bit.
Thomas Hardy - Longbow
Ralph Payne-Gallway - The Art of the Crossbow
2 Scienterrific American articles - one on bows and one on crossbows,
from the late '80's, early '90's (someone
borrowed them from me... grrr.)
Osprey's Military History books - oh, lots of 'em
oops, blanking on the author - The Medieval Art of Swordplay
Arthur Ffoulkes - The Armourer and his Art (iirc)
emails w/an honest to god, working plattner - "So, what about a longbow
arrow hitting your plate?" "Oh, it'd go right through it!"
Several books on ancient warfare - various authors have noted that the
various descriptions of the orders of battle have placed the slingers
BEHIND the sba (e.g. - Trajan's Column). They also describe the wounds
from the slings. Not pretty.
"Rocky" Russo - lecture series. He's also the author of "Achtung, Mustang"
(which is not relevant other than to give him some bonefides) - a WWII
air combat game, and "The Art of War", an ancients - renaissance
minis game. He also actually TESTS his work using replicas - as in
shooting xbows, throwing martio barbellae, and the like. Wish he had
a website...
Prime Mover: A natural history of muscle (have forgotten the author)