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To illustrate what I mean, let me give a quick example:

In The Dark Eye, there is a Staff Ritual named "Hammer des Magus" or "Hammer of the Mage". Its effect is that everything non-organic you touch with the tip of your mage staff will be thrown back with a pretty nasty force (the force depends on the stats, but at some point, we knocked over old trees with it, so you get the idea).

One of my players joked that he should get a metal tube, fill it with stones from one end and push the staff in from the other, building a primitive shotgun.

Now, luckily, he did not try to sneak that past me (the current GM), but if he would try, I have no idea if I should allow it.

The questions in this case (and in most similar cases as well) are, generally speaking:

  • Is it plausible that a character in a medieval setting, knowing nothing about modern-day science, will start building something like that?
  • If yes: Would you include any skill checks for it? There are some things that might fit in TDE, but they are usually so unbearably useless that no-one ever skills them (Which might be a good reason to ask for skill checks on them)
  • If you allow it, how do you balance it against existing game items and mechanics?
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up vote 15 down vote accepted

Since you asked for a general answer, and not one specific to your example, I'll put in my two cents:
(You didn't tag a system, and I don't know what "TDE" is, so I answered this medieval D&D-style.)

The Invention Process

Human ingenuity is unbounded, especially when it comes to new ways to kill people. The problem is that we as residents of the twenty-first century have a much greater knowledge of physics and math than most people in the medieval world. People knew how to use battering rams and slings, but not about Newton's Law that explained them. Archers could predict the trajectory of an arrow, but nobody could do the math to explain it. If a player can relate their idea to a well-known item in their current setting, let them do it. If what they're trying to replicate hasn't been invented yet, still let them do it, but have it require maybe a DC 15 Int check and/or 1d4 months of research. (In your example, it's pretty easy to look at a sling and think "stones moving fast hurt." It's harder to come up with the idea of directing the stones via a pipe - that might take a check or some trial-and-error "research".)

Using the Invention

Generally, MacGyver-ing weapons wouldn't require any sort of skill check, but the resulting weapon will almost always be improvised or exotic and take a penalty to its use. Maybe let the character gain proficiency with it, but only after a long time training. The exception is siege engines, which already have a Profession skill associated with it. Otherwise, require Craft checks—either as generic Int checks or in some specific associate skill—to successfully operate. I wouldn't create a whole new skill just for a player-devised invention. If anything, create a new Craft or Profession subskill.

Balancing Player Inventions

Balancing player inventions is even harder and more subjective. Note, though, that unbalanced things tend to tip over. If your 1st level PC figures out how to make a shotgun that destroys everything you throw at it, blow it up in his face. Don't be malicious about it, but make it clear that the character simply isn't skilled enough to use the powerful item reliably. Maybe in a few levels, if the character has invested a little more time into it, a better system can be developed. "But for now," say, "you can tell this particular invention has a high chance of failing catastrophically." In your example, maybe require a Dex check not to touch the staff to the pipe itself, which would throw the pipe and probably injure the character's arm (Dex or Str damage.)

Another good way to balance player inventions is to simply make them impractical. If the player isn't putting a lot of thought into actually developing something new, but simply says "wouldn't this be cool?", it's probably impractical already. In your example, it would probably be a full-round action to load the stones into the pipe, and another standard action to insert the staff into the pipe, firing it off. Let them do it, but they'll quickly learn that the combination of terrible rate of fire, poor range, and unreliability simply isn't worth it.

If, however, the player's putting a lot of thought into it, and their character actually invests time and money into it's development, totally let them do it! Maybe delay their research til it's a little more level-appropriate, but let them know it's feasible, just not right now. Balancing this is a little harder, and basically comes down to rewarding role-play. One trick I found is to make sure that the invention benefits the party, and not the character. If the entire party becomes slightly unbalanced, you can just up the encounters a little bit, and everyone has fun. If it's a character that's unbalanced, they take center stage and everyone else feels left out.

Once, I convinced my GM to let my high-level Gnome Wizard invent a telepathic network by creating a massive intelligent item which could scry and cast telepathic bond. (Basically Skynet.) I got government funding for it and developed it in my and my character's spare time, (our campaign had several skips of multiple years between sessions,) and they used it to communicate with their army's general and special task forces (including our party.) The GM balanced it by saying that my character couldn't become rich off of it—all profit was reinvested in what became a medieval CIA—and that it benefited the whole party. Plus it made a great plot point when Skynet decided it knew better than the government...

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TDE = The Dark Eye (Das Schwarze Auge, DSA): – Stephen Mar 13 '12 at 19:53
Thanks for your ideas about balancing. This seems to be a great way to go about it, and the next time a player proposes a halfway realistic invention, I might allow it and see how it plays out. – malexmave Mar 13 '12 at 20:42
@malexmave—Just be sure to make a clear distinction (both in your mind and to your players) between "inventing" something and "sticking two things together and seeing what happens." Reward hard work and roleplaying more than crazy ideas. – dlras2 Mar 13 '12 at 20:45

There's also the option of making them question whether they want to do it.

"There are a lot of wizards who can cast Mage Hammer. Some of them are smarter than you. If it can be used to make a magical shotgun, you're unlikely to be the first to think of it and, if it works, others will definitely copy the idea once they see it used.

"So, yes, you can try it, but, if it works, expect others to also use it against you."

Anything a PC can do, an NPC can do. And some of them can, most likely, do it better.

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I like your style. Never really thought about it that way. – malexmave May 2 '14 at 8:15
This is true, but I've seen it devolve into "rocket tag", where both the party and the enemies are full of deadly weapons. – Zachiel May 2 at 18:51
  1. Depends on the specific medieval period you're trying to model. Late medieval periods did have black-powder firearms. Projectile weapons were known (crossbows) to well-known (bows, assorted siege machinery), so the idea of launching small rocks from a tube using magical force doesn't strike me as being horribly out of reach. Would you react similarly to a PC wanting to launch large rocks, using the same spell?

  2. Yes, I'd require a new skill ("aim gravel-tube" or whatever the name turns out to be) starting from "abysmal", skill-level growing as a medium-difficulty skill (IF there's the concept of various skill difficulties) to centre the effect-cone on a certain point.

  3. As far as balance goes, there are a few things you can do. It's using already-existing "throw things using a mage's staff" mechanics, that probably drains spell-slots or spell-power of some sort and that doesn't go away. Damage-wise, let it be the same as rocks/gravel of the same size dropped from 30-odd metres, minus one metre of fall height for each metre of vertical distance between the tube and the target (I am blithely assuming that your system already has rules for falling rocks, it may well work to just substitute plain falling damage here). The specific height may need a bit of tweaking, I picked a number that gave some range, while still being effectively-lethal at point blank.

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I believe TDE uses point-based magic energy, but that's based on me playing Drakensang, so anyone's bet is as good as mine. – Kyle Willey Mar 13 '12 at 15:55
TDE indeed uses point based magic energy, so called ASP ("Astralpunkte"). For some spells, you can change the damage output by pouring more ASP into the spell. Hammer des Magus does not do any direct damage though, only the kinetic force, but I guess I could work out a formula for that, should it come to the invention. – malexmave Mar 13 '12 at 20:44

Answer 1: Probably, especially if your game is more of a late middle ages game. The middle ages lasted until the 15th century, and Europe had primitive guns in the 14th (and china had the idea in the 8th). I think it's very plausible that someone might think of the idea of using a tube to focus some shot.

Answer 2: I don't know TDE, but personally I would use whatever combat skill made the most sense.

Answer 3: Honestly, I feel like this particular firearm would be really, really bad. Think of how awkward it would be to shove a long stick into a shorter tube. Also, since he wouldn't have any of the technologies that made guns much better in later years, like rifling, the effective range of the gun would be very short. In addition, it's firing rocks instead of proper shot, which means what little accuracy he would have goes out the window. If I were running this in D&D 3.5, I'd give it ~1d10 damage in a 15 foot cone in front of the player. If you don't know the system, that's enough to probably kill a normal person, and enough to slightly injure a player character.

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To expand: canons were used at the siege of Seville in 1247. Complete suits of plate showed up around the first third of the 15th century. They're absolutely comparable levels of technology. And there's a reason they took a long time to supplant bows. – Blue Footed Booby May 1 at 21:06

I would invoke the 'Rule of Cool'. Is it's cool and awesome to turn a magic staff into a Shotgun? If the answer from your players is yes, then go for it!

Now, if you are playing a game with a high level of combat balance, rule that the 'shotgun' tube does not have the proper barelling to work perfectly as a gun, and then treat it as a shorter ranged but higher powered bow appropriate to your group's level. You could also bump up the damage by a step and rule that it takes an extra round to reload. This should leave the awesome, without screwing up combat balance.

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You don't answer the three listed questions (beside "yes, allow it"). – C. Ross Mar 13 '12 at 13:50
Our group tries to stay as close to "reality" as possible (if you suspend your disbelief about magic and all that stuff), so the Rule of Cool does not apply to that group. I however once played a self-made RPG where we at some point decided to introduce a new stat: "Matrix" - For extra cool actions like jumping over a fence while shooting a zombie in the face. There, the rule of cool was definitly applied. I however feel that it has no place in our group, as we focus very much on story and less on cool mechanics. – malexmave Mar 13 '12 at 20:46

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