[InquisitorHeidfeld]

How does that fit into UK gun law?  I know that beyond a certain air pressure of air rifle you need a Firearms License own/use it, but this isn't an air rifle, and probaby isn't even a conventionally acknowledged type of gun, so there might not even be a law concerning it.  I'm really quite interested in this one.

Convential acknowledgement is of course not required for a weapon to be illegal :-)

If your projectile were to be a dart-like form then you would run the risk of being classed as a blowpipe - a class 5 firearm in the same category as machineguns and fully automatic rifles.
Which a ball bearing as the projectile then you're in a better position, it is generally accepted that a weapon firing a spherical, non-frangible projectile at muzzle energies of 1 joule or lower are incapable of causing significant injury provided that appropriate precautions are taken (eye protection primarily).
Furthermore, 1 ft/lb is deemed the low limit for air weapons - projectile launchers below that limit are effectively toys.

Keeping the muzzle energy at or below 1 joule therefore would keep you pretty safe - particularly for a physics experiment.

(For reference:
1off, 6mm dia steel ball bearing = 0.88g

Muzzle velocity would have to be kept below about 47m/s to keep the energy in the 1 Joule range.)

[Adlan]

And 12ft/lbs is the limit for a non FAC air rifle.

[MarcoSkoll]

@InquisitorHeidfeld: Regrettably, a sphere is actually just about the worst shape for coilgun efficiency. An ideal is a solid cylinder, but that then has aerodynamic problems.

Usually, there's some kind of compromise with a cylinder with a more aerodynamic nose.

[DapperAnarchist]

... Like a bullet shape? You know, a bullet...

Though that may cause its own problems, legally...

[MarcoSkoll]

A spitzer bullet (like most rifle bullets) isn't quite the shape in question, and non-spitzer pistol bullets are too short for their diameter... but like a long pistol bullet I suppose.

Additionally, it's hard to rifle a coilgun (hard steel bullets don't take up rifling easily*), so it's really best to do them with a hollow tail so that they're drag stabilised - not that drilling down steel rod is all that easy.

*That said, with the right magnetic fields, it's possible to induce spin. Still tricky.

[DapperAnarchist]

well, the classic shape is what I meant, not a specific design...

[InquisitorHeidfeld]

Cylinders, particularly pointed and drag stabilised cylinders, though are too close to darts to be safe with regard to the blowpipe classification.
A sphere may not be an efficient projectile but it offers the least risk, legally speaking.

Yes, I've seen people using nails with the heads removed as their projectile - the trick there is that there's a heck of a lot of mass behind a very small surface area in that case (or any other involving a dart-like projectile) - it's not going to need to hit the eye to cause potentially fatal injury... It's unlikely to do so but likelyhood has no impact on that part of legisliation.

In legal terms you're on very questionable ground with anything over 1 joule, you're in incredibly tricky legal ground on anything where the projectile's contact area with the target is very small given its mass...
Ball bearings may not be ideal but at least there are defendable precidents.

Keep in particular mind that fact that any charges levelled would not be "possession" but manufacture, if the device was deemed to be a weapon then you'd have to swim quite a way to break surface, you'd be that deep in the brown stuff.
It is therefore that much more important to steer a wide course around the legal shoals. If the object is to understand or illustrate the principles then efficiency is unimportant - so efficiency is not a suitable defence.

[MarcoSkoll]

Cylinders, particularly pointed and drag stabilised cylinders, though are too close to darts to be safe with regard to the blowpipe classification.

You're mistaken here. Blowpipe classification only goes so far. It doesn't cover every and any dart like projectile, for arrows and crossbow bolts would fall under that definition, yet both are entirely legal. As I describe them, coilgun projectiles would even fall further outside being blowgun projectiles.

Blowguns are illegal because it's deemed that they are "a weapon of whatever description designed or adapted for the discharge of any noxious liquid, gas or other thing" - that is, they're designed for the purpose of delivering poisons or sedatives (even if they may not use them.)
A coilgun would be unlikely to be assessed as such.

Keep in particular mind that fact that any charges levelled would not be "possession" but manufacture, if the device was deemed to be a weapon then you'd have to swim quite a way to break surface.

The important wording in law is "designed or adapted as a weapon". There are many air cannons that greatly exceed energy limits, but are legal because they are not designed or adapted as weapons - they're special effects cannons mostly.

If a coilgun wasn't designed as a weapon, and you have evidence to that effect (mostly by demonstrating what it was designed for), you immediately have a defence. While my projects are legal (or indeed, licensed), there's reasons why I keep very complete notes, so if at any time, the fuzz should decide to change their mind, I have no end of things to throw at them.

[InquisitorHeidfeld]

The blowpipe effect is one of many in law which is trying to cover a "multitude of sins", if the CPS decide that your pointed, drag stabilised cylinder could be used to deliver a chemical charge then you're defending yourself against manufacturing class five firearms.

You have copious notes and you're fairly well covered - all well and good, but that doesn't apply to everyone. And while special effects companies can launch 5 gallon water bottles with ease, most people who build themselves gas cannon, spud guns...etc are running a serious risk.

If you give the legal hassles a wide berth then you have the option of using "because it was cool and I wanted to see if I could" as a defence - fall afoul of any legislation though and it's a different story.

Look at the VCRA of a couple of years back for an indication of what the law can classify as an illegal weapon, extreme examples included a child's transparent plastic waterpistol which classed as a RIF by the wording of the act.
I'm not saying that you can't build a 10 Megajoule railgun firing a fin stabilised, tungsten tipped dart as a personal project... I'm saying that to be safe you'd be best avoid doing so.
Avoiding all of the significant legal shoals means a spherical, non frangible projectile with a muzzle energy of 1 joule or less. More power, shaped darts...etc all increase the risk.

[MarcoSkoll]

If the CPS decide that your pointed, drag stabilised cylinder could be used to deliver a chemical charge...

Well, I wouldn't advise a pointed coilgun projectile (although some people will use them). It's better to have a rounded nose in most cases.
As I make them, I can't see that my projectiles would be considered an issue under law.

Anyway, we're now horribly, horribly off-topic.