by Malexmave Is it plausible that a character in a medieval setting, knowing nothing about modern-day science, will start building something like that?
YES And with Mages WHO DO NOT! live in Dirt Hovels and soldiers who posses even Bronse metal swords? YES. Concept from existing Tech's:
Any witnessed of a Bow or Crossbow or Ballista, or tapestry of a siege catapult used/seen before the characters. Any applied Use of "Hammer des Magus" into water at any Floating valid target (splash) rocks from lake/rive bed go up in your face. Any Use of Any armor type to Shield any party member from arrows or thrown rocks or "shrapnel" from Hammer des Magus.
THE First Test: touching an arrow that did not fire strait. The Desire NOT to Touch the Cross-Bow with TIP.!! The Desire to think "hay this could push 5+ arrows as easy if we could kept them on track". The Use of a Hallowed out U or V -ed Plank/Log. UP-Scailed to needing a chok-block to Arch it. Now, A Clay-JAR filled with Rocks.
Second TEST: "A round Metal Base Under A Clay Jar" Filled. The minimal Int Score to use a shield and fear that mages stick. The Desire to Use a Longer Tin sides on that Metal Base-plate & thin-Jar. Next: The Thought, Hey! IF Two Half U Tubes WERE tied together, We Could move this around Loaded. Then, Hay a Tin-metal tube is Lighter then Wood.
AMMO: Tin side-curved Bottom plate on ANY jar. Any Clay or Glass Pot/Jar/Vessel With a Seal-able top. Loading: Rocks Go in Jar, Arrows/Quarrels held tight in flax put in a book/Magic Scroll length Jar.
I've taken all this at a TL3-4 Tech level point of view, about Early Middle Roman/empire of era simple tech (if even that much adv.), using only Market grade and (semi-cheap) palatial house building products, for even the Ammo!..
Stick tech, One " Hammer of the Magi, aka 'de Force'Spitter" Stat powered, per Bearer & user of arcana Magica ect.
1 hammered tin-metal tube, Mildly expensive, indoor fountain pipe w/ Very thin copper poured on tubes "wield" spot, Variable size's SEE: Ammo. If you have a Even a Tin metal Sword the tech exists for the tube making -- with out magic effort assistance. Sand open air molds, A Hammer And Rock to bend it onto rounded shape, a smaller rock for the tighter shape smaller is desired.
by Malexmave 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 skillchecks on them)
Aiming? Arch Angle trajectory Only for extra MAX range (mild experimental). Not needed at under 40 feet, Aiming Skill is more based on Trusting the Mage (fear check) As he touches the staff to the JAR. (IF STAFF TIP TOUCHES THE TUBE, PEOPLE GET HURT!) Tube on your shoulder. Now Armor Penetration is based on Shape, Arrows and spear tips are known...
by Malexmave Again, if you allow it: How do you balance it against existing game items / mechanics?
*WHAT is this NEW ITEM HE Proposed? A ROCKET LAUNCHER Zero Recoil, Zero internal "tube" Pressure from (gun powder) gasses. No need for Rifling Tech on a single or multi Shrapnel/Quarrel/Spear Rocket/missile. Encumbrance Based on Diameter and thickness choices of Loaded Wood/Metal Type Mass.
Dangers of Use? Well.. Depends on the PHYSICAL Weight of mass Being displaced (ammo) By the Confident User of Said staff-tip, Hour to Hour Consistency of his Magic Stats (or magic power input into Staff-tip force) And a Steady Hand.. & the Longer from Tube rear Ammo sticks out, the safer for all. ADD a small Wood notched pegged lever in the tube to hold ammo from backside on the Jar, most Basic of lever concepts.
Damage: Is by number of shots per load, TYPE OF AMMO/Ammo Mix, ODDS of STRAIT FIRING ARROWS/Spears IS Kinda HIGH for upto 1-5 items, as it could Start to spread a bit More depending on how wide the barrel/tube is bigger then the loading end.
Player: "OF Course, 2 Different tubes Types is better then 1." Pretty much Make a Missiles Hit Spread Table per Target (Battle Tech style), likely the first 20-30 feet is strait and after that, spread. Unless they go Wide-Bore Cannon right off the bat. Then the D&D Cone-Shaped spell structure works on both Hexagonal and Square maps. Force to mass, Distance per mass.