Make enchantments on launchers relevant without making them overpowered by adding their enchantment to damage (because projectiles already add their enchantment to damage): the enchantment determines the likelihood of getting +1 to multishot, unaffected by role or skill. This chance is given as either enchantment/(enchantment + 1), or enchantment/8.

If the enchantment is negative, the same formula is used on the absolute value of the enchantment to determine if multishot should be decreased by 1. If it is decreased to 0, you fail to shoot anything and waste your action.