From Project: Redcap
[anchor:Attack]Attack Total for Missile Weapons
The Attack Total for missile weapons is calculated using exactly the same formula that is used for all weapons (see page 171 of ArM5). This represents a change from Fourth Edition, where the Attack Total for melee and missile attacks incorporated different Characteristics. In Fifth Edition, melee and missile attacks use the same Characteristic.
[Anchor:Defense]Defense Total for Missile Weapons
Calculating the Defense score is a little special. First, the Ability to be used is the weapon skill appropriate to the weapon the defender is wielding. If he has a staff, he defends with Great Weapon skill; if he has a sling, he defends with Thrown Weapon skill. The explanation is that weapon skills include the ability to avoid being hit (see the last sentence under "Missile Combat" on page 172 of ArM5), though meta-game reasons (simplicity) probably play a large part in the real explanation.
Note that only shields, not other weapons, count toward Defense total against missiles. So against missiles, your Defense total would be Quickness + Combat Ability + (Shield Defense Bonus) + stress die.