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This years story of our fantasy combat LARP features the siege of a city. Eventually this will culminate in a battle, where there is an actual siege of a palisade-type fortification, including siege weapons (we have done this successfully several times in the past).

This year we had the idea to include a dwarven party supporting the agressor: by supplying them with a dwarven siege cannon. What we're still struggling with, is how to make the cannon safe, and fun.

Requirements:

  • It must be safe for our participants - for combatants and the crew manning the cannon.
  • The cannon should shoot real projectiles of some sort
  • The cannon will not shoot at people deliberately, but will instead be used against the walls/gate.*
    • Of course it must still be safe if one of the combatants were to be hit accidentally!
  • A range of, say, 30-50 meters would be nice (could be somewhat ballistic).

Bonus:

  • If at all possible the cannon should make a loud booming noise when fired
  • It would also look cool if there was a nice puff of smoke when fired.

How could we build a cannon which meets our above requirements?

A good answer would include:

  • Experience with cannons (or other "gunpowder" weapons) in LARP
  • What projectile is used
  • How it is propelled
  • How the cannon could be built
  • How special effects could be achieved (sound/smoke/...)

*) The idea is that after X hits with the cannon the gate/some section of wall will "collapse", i.e. be opened manually.

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You could create something like this: youtube.com/watch?v=3yc3fAC5vOI .The guy uses whistling nerf rockets to fire. – Lyrion Mar 16 at 13:34
    
(I've never done anything like LARPing before, so take this with a grain of salt.) ​ It would likely take significant effort as well as [money per ball], but if you really want the projectiles to be balls, then I would recommend foam dodgeballs, perhaps with the labels painted over. ​ From my brief search, the highest quality ones were these, and these were the cheapest of appropriate color. ​ (black/gray/silver/white) ​ ​ ​ ​ – Ricky Demer Mar 16 at 14:13
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I have a feeling you're going to run into the age-old "distance requires momentum; momentum creates danger" issue. Basically, you need enough velocity to fire the projectile the desired distance and enough heft to the projectile that it remains aerodynamic at that speed. – Wesley Obenshain Mar 16 at 14:55
    
@WesleyObenshain: You're of course right, but in my opinion LARP arrows or bolts already show us that these problems can be solved in an safe and fun way. – fgysin Mar 16 at 15:04

What you could use is something similar to what I've used in airsoft; an RPG launcher which uses a 40mm gas grenade to fire nerf rockets.

We used these for attacking vehicles (One of the players had a BMP-1) as well as land rovers.

It should be pretty easy to replicate one of these things; you basically need a tube that would fit a 40mm gas grenade that you then use to fire whatever missile you want, there are quite a few videos on youtube of these things in action so you can see the range of them.

The bonus of these things is that they're safe, no gunpowder or explosives to worry about, just a sealed propulsion unit that you use to fire whatever you have stuffed down the barrel. The range may not be what you need however, so have a look at the videos.

My friend hand-built a PIAT that used these and it was a tube with a slot in section at the end where the grenade was dropped in, (a whole tube section pulled out) you then sealed the tube (rubber o-rings to seal, the important bit is to make sure that the seal is tight so there is no gas leakage) and used a basic trigger to fire it; you could wrap a cannon around that.

You won't get a burst of smoke from that, so it won't help with that; but for things like that we generally used blanks (for artillery pieces that were just for show) so if you rig the trigger up to fire the grenade as well as the blank you'd have smoke and a projectile at the same time.

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Flour can be explosive, so I'd be pretty careful about that ;) en.wikipedia.org/wiki/Dust_explosion – Rob Mar 16 at 10:46
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Fine, sieved ash is probably your safest smoke generator. Be sure to only ever use proper, long-lit bonfire/fireplace ash - BBQ ash often contains enough unburnt charcoal to warrant a hefty explosion. – eimyr Mar 16 at 13:33
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I would specifically recommend a TAG 40mm airsoft launcher, just make sure you get the dummy rounds and not the rounds that explode on contact. – Probst Mar 16 at 14:24
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My google-fu has failed me... Can someone explain to me what these 40mm gas grenades are? How expensive are they? Can they be refilled? – fgysin Mar 16 at 15:42
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They are shells for a 40mm grenade launcher in paintball and airsoft, like M203 or EGLM. They are triggered with a simple pin pushing a valve in the back so you do not actually need an airsoft M203 to use them you can make your own tube. The popular ones for launching projectiles are TAG shells which are like $50 for the reusable shell and projectiles (The non explosive ones like you'd want are also reusable until they deform) are like $50 for a pack of 10. You fill it with green gas which is also an airsoft product, one canister is about $15-20 and should power a good 30 shots. – Probst Mar 16 at 16:59

When I used to do pirate LARP we used the guts of a Nerf gun for this. Someone had built or acquired a little gun carriage and dummy cannon and we put the Nerf launcher plus (painted) rocket in it, with a little talcum powder inside the rocket for smoke effects. Pulling a string launched the rocket.

The advantages of this are that the firing mechanism is cheap and it does actually fire a projectile that everyone can see.

The disadvantages are that you need to build yourself a prop to make it look like a cannon and it doesn't go 'boom'.

We tried using the bangers from party poppers inside something that echoes to amplify the sound, but really if you want something to sound like a large 'boom' outdoors then you're going to have to resort to something that makes a large boom, and time your Nerf-firing appropriately.

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What kind of range do these Nerf launchers have? The missile looks quite nice actually... – fgysin Mar 16 at 13:13
    
Our little ship's gun was only ever used at a low angle to fire at melee attackers, so I don't know how far it could have gone. It certainly went a good couple of dozen feet. – Nelviticus Mar 16 at 15:32

If you want a cannon that fires a ball, then you might look at using something based on the crossbow type technology- basically a bungie catapult in a pipe. This would be reasonably easy to construct and to prototype - it would certainly be worth prototyping to ensure that you can create the amount of safe motive power you want that way.

For a projectile, I would be thinking of a soft foam ball of the lightest type you can find- furniture foam would probably be best if it is going to be flying through the air at speed. A layer of latex would allow you to give it colour and allow it to fly a little better without impacting safety.

Whatever you use, if you're firing it at people you need to first do the maths to ensure it's within safe parameters and secondly test the hell out of it to absolutely minimise the risks to targets and the people firing it.

One other suggestion that might give you the range and the effect would be to have a simple powder cannon that only gives you a bang and smoke ( still don't want to fire it too close to anyone! ) and then have a ref using a regular LARP-safe longbow to physrep the projectile. It won't look like a cannonball, but it will be very clear where it lands/what it hits and a bow should have sufficient range for the job. The other option there would be to have a ref on the player's side calling the impacts for them. If you look at some of the systems that use black-powder weaponry this approach is not uncommon and breaks immersion less than you might expect.

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A lightweight foam ball isn't going to have the aerodynamics to go 30-50 meters. A foam soccer ball might be dense enough to do the job, but you'll need to make sure any combatants are well clear of the business end of the "cannon". – Mark Mar 16 at 22:30
    
This is the problem with this set-up in general - anything that can travel far enough is likely to carry enough momentum to really suck if it hits you close up. Depends a lot on the context and the players, but when in doubt it is worth erring in favour of safety. – glenatron Mar 17 at 12:28
    
Never built one myself, but I saw a cannon built out of a large PVC pipe using waistband elastics as a propelling mechanism. The cannon fired small balls, not unlike those used to play dodgeball and the like, so they were heavy enough to travel quite far, but not hard enough to really do any harm. Accuracy wasn't so good, but not against a large pallisade. Paint the ball silver/metallic and use talcum powder for smoke effect, and you've got a nice cannon, once properly decorated. – Dungarth Mar 17 at 13:50

I'd recommend a vortex cannon, something like this:

http://makezine.com/projects/build-giant-collapsible-vortex-air-cannon/

You could make a smaller one like this, which is also styled in a way you might like:

http://www.instructables.com/id/Potent-Pirate-Smoke-Ring-Cannon/

You said "siege" so I assume you want something big- bigger than a handheld man portable device. This would be physically large, but not unwieldly as it can be dissembled.

The projectile is simply air- to make it visual, you can use dry ice to make fog which would make it visible (or if you have power you could use a smoke machine). It will be harmless, non toxic, but also clearly indicate when something is hit because the smoke ring will dissipate and splash around the target.

Some say the range of these cannons can be the 30 meters you want, but on a chaotic battlefield... could be quite variable and heavily dependent on weather conditions. Rain will probably render it entirely ineffective, as will strong wind.

The primary advantage of this is that it is doable at a hobbyist level, uses cheap and widely available materials, is easily replicated, and is entirely, completely, provably LARP safe.

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I have no experience with this, but my overactive brain came up with an idea very quickly with this one :)

For starters, I'd recommend steering toward a rocket projectile rather than a classic "cannonball". At the safety level required for the velocity, softness and size of the projectile, a rocket offers better aerodynamics.

If you want to keep the cost and complexity down, you could go with something similar to Rob's answer and cannibalize a Nerf gun of the appropriate sized projectile (Nerf proper used to do a "bow and arrow" which was essentially a finned rocked on a cockable launcher. You could pick this or something similar up off Ebay and hack off any extraneous bits with a hacksaw. Other suppliers probably still do "RPG" type launchers which might also be useful), place it inside the "cannon", and then add some noisemakers and smoke.

You could make this as complex or simple as you like. Heck, this could probably be rigged up with some cardboard, your projectile launcher, a speaker rigged to someone's phone or a small computer for sound effects, some LED's and a smoke machine! Firing the nerf gun could be done with a string rigged up to set off all three at once.

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-1 Starting with 'I have no experience with this' to a question specifically asking for experience – Wibbs Mar 16 at 11:23
    
Caveat - Guy is saying a good answer would include experience, so I've caveated it. Doesn't stop the idea being useful. – Miller86 Mar 16 at 11:35
    
Your answer doesn't seem at all definitive and is more like a brainstorming response to a forum post. – LegendaryDude Mar 16 at 13:33
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Regardless of the poster's requirements, we expect all answers on RPG.SE to adhere to the Good Subjective, Bad Subjective criteria (blog.stackoverflow.com/2010/09/good-subjective-bad-subjective). – mxyzplk Mar 16 at 13:41

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