Missile file in combat has to be treated a bit differently from normal combat. It is broadly equivilent to hand to hand combat, apart from:
Use BS rather than WS
The weapons effective strength is used rather than the strength of the attacker.
(However some weapons use the attackers strength, like spears, etc)
You need to load the weapon (usually) as well as drawing it. In some cases this
can take place in a different round from aiming/firing.
You need to aim the weapon, which might take longer for firing at extreme
distances.
There are modifiers for firing at different ranges; for each weapon there is
given the distance for short, long & extreme distances.
To hit modifiers:
Firing at a small target (under 1' tall/long) : -10
Firing from a moving object : -10
Firing at long range : -10
Throwing improvised weapons : -10
Targets in soft cover (vegetation, etc) : -10
Targets in hard cover (partially behind a wall, etc) : -20
Firing at extreme range : -20
Extras:
Firing at extreme distances takes a round longer to aim/fire, unless the GM
discounts this (ie firing into a crowd, etc)
Using a specialised weapon without the relevant skill : BS = 10
Sharpshooting: aiming for a part of the body - counts as a small object
(see above), takes a round longer to aim/fire, if misses will miss the
opponent all together (unless in a crowd, etc)
Can get rid of draw/load time by carrying around a loaded weapon. Gets a
bonus to I of 10 for the first round of firing
Bombs/incendiaries: if you miss, the bomb will land 2D6 feet away from the
target in a random direction (roll D8 for compass direction)
Bombs/Gunpowder weapons: for the hit roll, if you roll a double (ie 66, 33) is
is a misfire: for gunpowder weapons (pistols, etc) can fail to ignite (may or may
not have to reload) or may explode causing automatic hit on user and destroys
weapon. for bombs can fail altogether, could splutter and explode at a random
time later on, or explode either half way to target or in thrower's hand.