Useful bullets are dense. X-rays, essentially, scatter off density. Building a useful bullet that's relatively transparent to X-rays may be an impossibility.

Rubber bullets have killed people before, and I reckon rubber bullets would be as deadly as any other at close range.

Good point, and a blank is deadly at contact range. And the gun is aptly named after the WWII Liberator: http://en.wikipedia.org/wiki/FP-45_Liberator

The objective of such crude and limited guns is to enable you to procure better ones, although of course this one is also an initial proof of concept, plus I gather intended to work though such issues as this takedown (I've read the guy or people behind this believe they've jumped through the required hoops to allow Internet publication).

Although I'll note conventional ammo needs a metal case; there has been work on caseless ammo but it's not gotten beyond R&D, the G11 probably went the furthest: http://en.wikipedia.org/wiki/Heckler_%26_Koch_G11.

But you can't fire a rubber bullet from a plastic gun. That gun as designed needs a propellant (cordite/gunpowder) to fire the metal slug at the end. And if you need a propellant, you can't use a plastic cartridge case.

If you have a plastic "thing" firing a rubber projectile using a plastic spring or similar, then it is more like a crossbow than an actual gun.

Why not? I bet I could cast the charge out of an explosive resin.