Actually, Jonah, railguns frequently use aluminum projectiles in real life.

There are many different ways to magnetically accelerate projectiles.

Coil guns use a series of electromagnetic "donuts" around the barrel to successively yank a projectile down the barrel. The projectile has to be responsive to magnetic fields - the easiest way to do this is to make it out of iron or some other ferromagnetic alloy. Aluminum won't work, as you noted.

I'd like to make a point about coil guns, though: their electromagnetic coils are probably not made of a magnetic material. They probably use aluminum or copper windings, neither metal being magnetic. They're turned into magnets by running electricity through them. (Basic fact of electromagnetism: where electric current flows, there is a magnetic field.) Did you ever do that in science class or at home? Take some copper wire, wind it around a pencil, and apply a 9-volt battery, then lift a nail or some other piece of iron?

This brings us to rail guns. Railguns don't give a wet fart about how the projectile material responds to electric fields. The important part is that the projectile is CONDUCTIVE. See, railguns actually generate a magnetic field in their projectile by running an electric current through it. The current from capacitors is run up one rail, across the projectile, and down the other rail to complete the circuit. Because the projectile's electrical current is at right angles to the electrical currents in the rails, it's repelled by the magnetic fields in both rails. Aluminum makes a great material for the railgun prototypes because it's very electrically conductive and easy to shape.

(Note: aluminum is a good material for railgun prototypes, but it sucks for actual weapon applications. It has abysmal properties for a penetrator.)

Here's some links to real life rail guns that use aluminum projectiles. There seems to be a whole rail gun hobby culture out there:

Copper, Aluminum, and Graphite Projectiles

Aluminum Projectiles and Railgun Theory

Blog of Railgun Construction, with Aluminum Projectiles (Called "Armatures." Also some railgun theory.)

University Class Discussion of Railguns, with Aluminum and Graphite Projectiles

That's an excellent suggestion. I like the idea. The players get their rail guns and the GM only has to deal with a slightly weakened version of an existing weapon.

Rifts?
/shudders
Palladium Game's products in general!!
i.e. Robotech, TMNT, Ninjas and Superspies, Heroes Unlimited, etc...

Yeah, they do...but actual rock (or the just silly "balls of garbage" described in T:WL) is a poor choice for orbital bombardment. Plain silicate rocks smaller than a hundred meters or so in diameter just don't survive re-entry, not intact. They fracture and break-up.

Metals are a good choice. Iron's a good, cheap, common material for lunar mass drivers to sling at Earth. A large slug (10+ tons) of solid aluminum would work, if you're willing to accept some mass loss.

If you're talking proper, purpose-built orbital bombardment weapons (a real Thor), you just need to get fancy with ceramic or ablative heat shields, careful aerodynamics, etc.

That is what shock obsorbtion is for.

Orbital flechettes: Thor as portrayed in Niven and Pournelle's "Footfall."

There won't be a lot of environmental consequences out of any impactor less than a few thousand tons, and even then they'll be local "small quake and lots of dirt raining on everything" effects.

You need to get up to objects a hundred meters across (1-3 megatons of rock) to have regional consequences from the impacts.

That air contamination involved all sorts of fun, early 1970s building materials, plastics, asbestos (fire protection on the lower floors), etc. that wouldn't necessarily be found in normal soil.

Of course, if the orbital shots are hitting urban areas in Shadowrun, areas that make the 1970s look like models of environmental protectionism...and, gee, the good SR targets are all in urban areas, right?

So I'd have to pull a bunch of numbers outta my ass to really argue this, but - just what sort of shock absorbtion are you talking about? Even a quite light object moving at those speeds - say, the weight of a 7.62mm rifle round, about 10 grams, and let's say 2km/sec - that's, what, around 14,000 foot-pounds?

I would like to mention here that the numbers above are arbitrary. The point I'm making is that just about anything carries a tremendous amount of energy as velocity picks up. Remember that KE = .5 * m * v^2, and that squared part pays off.

Note - please, someone tell me if I'm on crack. I've done the math but I don't actually know that much about my starting assertions, and they may be utterly irrelevant.

heinlein ++

an excellent book!

Is the Melta a microwave weapon as suggested above? (<--Question from someone who's played WH40K once.) If so, vehicle armor should stop it easily, as should any conductive body armor.