Missile Weapons

The advantage of bows is rate of fire (and sometimes range).

The advantage of crossbows is ease of use, and - for larger models, armour piercing.

The advantage of slings is ease of carry and ammunition supply.

The advantage of spears is weight (and therefore damage).

So, let's lay this out:

Range modifiers: 0/-3/-6/-9

Weapon	Base Attack	Range in Feet	Armour Reduction	ROF
Bow (Longbow)	-4(-6)	30/60/120/240 (30/60/180/320)	1/0/0	1/round
Crossbow (Heavy)	0	30/60/120/240 (30/60/180/320)	1/0/0(2/1/0)	1/2 rounds (1/3 rounds)
Sling	-4	20/40/80/160	0	1/round
Spear	0	10/20/40	0	1/round

You can get rid of the penalty for using a bow or sling if you're specifically trained in the use of bows or slings. For a fighter, that means forgoing training in one of the other weapon types (daggers, swords, axes/maces, polearms). For another class, it means giving up a comparable benefit (I'll be generalizing the classes soon, and so this will make a little more sense).