To determine what weapons to carry, consider your situation first, and then your skills, strength, and budget. If you can't use it or don't need it . . . don't buy it.
- GURPS Basic Set, p. 267
When choosing weapons in GURPS, generally people care about a few combat stats - damage, parry, weight. A few people might even consider cost*, or a coolness factor, or characterization. But usually falling by the wayside are two other stats that can be critical - LC and CR.
Legality Class (LC)
Legality Class (LC) is how the weapon is viewed by the legal authorities.
"Consider what the law allows, too. Most settings have laws or customs that govern the weapons and armor you may wear on the street or on the job without attracting attention (see Legality Class,box). This applies in historical settings as well. A stranger visiting the average medieval village wearing a suit of plate armor would be every bit as conspicuous – and threatening – as a person carrying an assault rifle into a corner grocery store today!"
- GURPS Basic Set, p. 267
Okay, sounds good, but Basic Set also makes the blanked statement that "All melee weapons and muscle-powered ranged weapons intended for combat are LC4." In anything short of a very controlled society, this means anything from a dagger up to a halberd and anything in between is fine.
After all, LC4 means the "item is openly available in most societies, but tightly controlled societies might restrict access or use."
So let's talk restrictions.
Setting a Control Rating (CR)
Control Rating is how restrictive - or not - your society is. It ranges from 0 (anarchy) to 6 (total control). It often varies by category - some societies are more permissive of weaponry, others of speech, etc.
So a strictly by-the-Basic-Set way to do this is to set a CR, and then either change the LCs of the weapons, or change the CR by class of weapons.
Aw, do I have to? No. CR just lets you systematize it. But with the default for all low-tech weapons being LC 4, you need a per-weapon stat or a per-weapon decision. You can skip right to writing text that says "Who can use this legally?"
This is what GURPS Basestorm does.
Megalos in Banestorm is a great example of this. Arms and armor are regulated by Status, not strictly by LC and CR. The concepts of LC and CR are there, but they are set out by weapon/class of weapons.
For example, bladed weapons are prohibited entirely to beggars or serf. You need Status 0 for a sword, you need to be in the military or the guard to have a military polearm. Missile weapons are prohibited to anyone but the military.
It's just weaponry, but also magic and armor.
You need to be a knight to own plate armor or magical weapons, and even mail and scale are limited to the armed forces. You want metal armor and a +1 Sword? You need to be nobility.
As written, status can act effectively to mitigate legal controls around weaponry.
Megalos being a corrupt state, you can pay bribes for special exemptions to these rules (and naturally, high-status folks can provide writs to their personal troops). Minor exceptions are made as well, and you can imagine these being enforced about like speed limit laws on US highways - when the police feel like it, they'll swoop down, but generally will ignore minor offenses.
So to be a typical adventurer wearing mail, scale, or plate, with magical weaponry and so on, you need to either pay some hefty bribes to get legal exemptions, or you need to be Status 2. Only the nobility can truck around with weapons.
Using this in your game.
It's easy to dismiss these restrictions - "If the players want to run catgirl ninjas with two-handed swords they should just be able to." Well, yes, and the happy feudal anarchy of a default D&D world, where anyone can own any weapon and walk around fully armed without any social cost is fine, too. But there are advantages to putting in social/legal weapon controls.
- niche protection. It's hard to upstage the knight at, say, heavy infantry tactics when only the knight has the status to use certain weapons.
- class protection. This can help explain class restrictions. Why can't mages carry swords? Not legal. They're effectively part of some restricted social class.
- weapon value elevation. What if composite bows are restricted to the nobility (think early period samurai, say) or only the rich can legally carry crossbows? Suddenly you've restricted "common" foes to more common weapons, making these weapons immediately more valuable within the environment.
- makes a variety of weapons valuable - weapon choices become more than just "best combat stats" but also "best legal weapon for my social class."
- acts as a resource drain - you can sell off, or trade away, that sword you found, or pay for a permit to use it! This can effectively drive further adventures.
- makes status a tangible, valuable goal. You don't want to be a noble just to be a noble - a vanity purchase in many games - but also because it allows you to arm yourself as you choose (and perhaps, issue permits to your friends). It has direct adventuring value - even combat value - by allowing you access to the best weaponry.
- it can narrow the weapon choices in certain adventures - in cities, in restrictive countries, etc. Or more accurately, force PCs to engage in smuggling, disguising, and otherwise concealing weaponry. This can add a big twist to an otherwise straightforward adventure, if folks have to rely on backup weaponry or "status appropriate" weaponry instead of whatever they specialized in because it's the most combat effective.
This can easily be pushed too far, though. Laws and restrictions have to drive two important things: fun and adventuree. If having the restrictions makes for a more fun adventuring environment, whether from "everyone has something to do" because not everyone is socially ranked for all weapons or because the need to get loot for paying for permits and bribes or to try to status-climb. It's not going to work in a place like, say, Krail's Folly (although it might, with a tweak), or in a purely beer-and-dungeons game like my own.
But for a game where social climbing is encouraged but might seem a bit too "endgame"-ish, the idea of legal restrictions on weapons and armor, and a need to socially climb to make yourself fight better, might be just the thing.
So GMs and PCs alike can really get some mileage out of the legality of weapons instead of just their combat stats.
* Although this is often overlooked for modern weapons. "Why would anyone use a weapon other than (X)?" is a pretty common question, but if weapon X is extremely expensive with expensive ammo, well, that's why right there.