It's time for Melee Academy. For the other posts in the series, check these links:
Melee Academy: Muscle-Powered Ranged Weaponry 101
Melee Academy: Entering a Room in Dungeon Fantasy
Melee Academy: Someone Took My Lucky Charms - Now What?
When you use a melee weapon, GURPS assumes you retain control of it - either you hit your original target, or you don't, and that's that. Barring a critical failure, of course.
But with a ranged weapons, it's possible to hit the wrong target. Either one between you and the target, or one on the far side. Let's take a look at the rules (from B389-390) and their implications for offense and defense.
Rule #1: Firing Through an Occupied Hex - Just because they're in your way, doesn't mean you can't shoot around them. But there are risks.
Each target (technically, "anyone") between you and your target gives you a -4. This is cumulative, which naturally means it's really hard to hit someone beyond cover.
Remember that prone, kneeling, or very small targets don't obscure you, but big ones can very easily fully block your line-of-sight (LOS). But it takes a standing SM+2 character to block a SM+0's LOS completely. SM+1 (such as your barbarian friend or the evil wizard's ogre bodyguard.)
But you can shoot past your friends. If, despite all of your penalties, you manage to hit, you hit the target you aimed it. Fair enough. But, hey, -4 is pretty steep. What if you miss?
Rule #2: Hitting the Wrong Target - If you miss you attack roll, you might hit someone else. This is bad if you're shooting past your friends. This is ten kinds of awesome if you're shooting past your enemies. Consider it a second chance to get some killing in.
Your chance to hit another target isn't great - your actual chance to hit that target, or a 9, whichever is worse. Plus you roll in order away from you - starting with the closest person (did you just shoot your friend in the front rank in the back?) and going all the way out to the maximum range of your shot. In a low-tech setting, especially with skirmish-like troop densities, this generally means only a few chances to hit. But with a long-range attack in a crowded area, it could mean a lot of potential targets to hit.
Rule #3: Overshooting and Stray Shots - Further, if you hit your target and the target Dodges, the ranged attack keeps on going. You start to check on the far side of that guy to see if you hit anyone.
This is why front-line fighters shouldn't Dodge when protecting thinner-armored friends. You're potentially letting them get hit instead of you. They may be "safely" out of reach of enemies and making All-Out Attacks with their own ranged weapons, but you Dodge that arrow and now they're a step down to the road to being a pincushion. Nice. Don't be that guy.
Now, tactically, what to do with this?
Use it to your advantage . . .
Take advantage of hitting the "wrong" (or as we say, "bonus") target.
Position yourself. If you can shoot in enfilade, do so. That will give you a chance to hit your intended target, anyone between you and the target, and anyone behind the target.
Basically, you want as clear of an attack on your chosen target as you can manage, but with as many enemies behind him as you can get just in case you miss.
That's if you have a specific target in mind. If it's a group melee and you only care that you hit someone, aim for the closest guy . . . but try to get an angle that lets you hit someone past him if you miss or he dodges.
This makes enfilade fire a great choice for lower-skill shooters. It takes you out from behind your own friends, and lets you get lots of chances (albeit, not very good chances) to hit your enemies as they're fixed in position by your friends. GURPS doesn't have Zones of Control or Attacks of Opportunity, but generally it's costly in terms of movement and difficult to disengage from melee to try and rush to a flank. Not impossible, but not easy, so shooting down the ranks from a flank from a moderate distance can keep you safe. Maybe not terribly effective, but if you've got a low skill, shooting from closer in but taking a -4 for shooting past your friends is more dangerous than standing at 10 yards away (also -4) on the flank. If you miss, your friends shouldn't be in your line of fire.
. . . but don't try to get cute - Don't try to aim for the "wrong" target to try and take advantage of these rules. It generally won't work out. You won't get a better chance to hit by accident than you would deliberately; the rules are explicit on this ("a flat 9 or the number you would have had to roll to hit him on purpose, whichever is worse.")
So you basically can't exceed a 9, or exceed whatever lousy skill you had to hit the target in the first place, by aiming at the "wrong" target. GURPS doesn't work that way.
Remember your goal - hitting the right target with the shot. All of this is great but bonus; if you move around to get a "better" enfilade but it costs you the Move and Attack penalty (for a normal guy) or costs you your Acc (for a Heroic Archer), it's probably not a great tradeoff.
But can't only Heroic Archers really take advantage? No, not really. A high-skill archer won't miss often, and can so easily shoot from behind allies these rules won't come up much. If you have Bow-22 and a Balanced Composite Bow (Acc +4) and Heroic Archer, you'll easily eat a -4 for your friend in front and -4 for the bodyguard in front of your enemy and still have an 18 or less to hit before range penalties. Your main concern is using "Overshooting and Stray Shots" to your advantage because of enemy Dodges.
Long story short: You can hit the wrong guy, so if your skill isn't great, line up as many "wrong guys" as you can in a row and shoot the closest one. If your skill is great, don't worry about these as much, except for that bit about hitting people past your Dodging opponent.