Missile combat is very similar to melee combat, but is distinct in several ways. This system can be used for anything from a thrown rock to a bullet to a space-based laser, and it should actually be possible to play golf, baseball, tennis and so on with these rules. Anything which fires a projectile is generally a missile weapon.
Missile weapons have greatly varying rates of fire (RoF), from the very slow (early cannons and muskets) to the very fast (continuous laser beams) and everywhere in between. Each weapon description (see the Equipment chapter) gives a rate of fire, either in jiffies per missile fired (for slow weapons, such as bows), or missiles per jiffy (for fast weapons, such as fully automatic guns). (We generally use the term "missiles" instead of "rounds", as a round is used elsewhere as a measure of time.) The weapon descriptions also give information on the flight speed of the missile (generally in metres per jiffy).
In the case of weapons which take several rounds to fire (i.e. rate of fire is given in jiffies/missile), the given number is the WS, and attacks are handled in much the same way as melee combat.
In the case of weapons which can fire several missiles in a single jiffy, treat the weapon as having a WS of 1, but with multiple chances to hit in a single jiffy. Each missile should be counted as a separate attack, so there will be several to-hit rolls, and possibly several damage rolls. The weapon description will indicate to what extent it is possible to fire at different targets in a single jiffy with the weapon, and what modifiers may be applicable for sustained bursts.
The target of a missile attack may make a defense roll, if the GM determines that it is possible, and if the target chooses to. The target may not be given a defense roll for many reasons (surprised, too close to the attacker, busy with another action, tied down, already dead, inanimate object, ...). The target must make a defense roll for each incoming missile (including those that would otherwise miss; a critical failure might indicate that the character dodged into the path of the missile).
A defense roll is a reaction to the action of the missile being fired, and so happens in the jiffy following the firing; if the missiles arrive before that time, no defense roll is possible. This is often true for bullets, and always for lasers (since the "missile" is travelling at the speed of light, the target can't tell where it's headed until it arrives). Pre-emptive dodges may take place, based on the aim of the shooter, but these are not defense rolls; rather, they are separate actions, which the shooter can react to in turn.
Which attribute or skill the defense roll is made against will depend largely on the circumstances. A person will make their defense roll against their maneuverability rating (see the section on Movement in Other Rules); a jet fighter pilot would roll against his piloting skill. In case of confusion, the applicable attribute or skill should be determined by the GM (and make a note of the situation and solution, in case it happens again).
All other information you need will be given in the description of the weapon.
There may be modifiers applied to missile attack rolls, depending on the circumstances.
A sniper has a rifle. His chance of hitting is normally 60. There is a telescopic sight on the rifle which raises his chance to 80. The GM rules that due to the distance, and the facts the target is moving and the sniper has to shoot through a plate glass window, there is a -30 modifier, bringing his chance down to 50. If he were to aim for one second (two jiffies), his chance of success would be increased by 10%(=5%x2) to 55(=50+50x10%=50+50x.1=50+5). If he were to aim for five seconds, his chance would be increased by 50% (the maximum) to 75(=50+50x.5 or 50x1.5).