> Given that a 3D prototype will be made of a totally different material
> with totally different properties, I'm not sure how having a prototype
> lets you check this.

As I said, the materials used for 3D prototypes are designed to mimic 
real plastics. The properties are very similar (especially for stiffness).

>> FWIW I've *never* seen tooling get
>> made exactly right first time, there is *always* something that needs to
>> be fixed or changed
>
> Really? I find that quite surprising. (Then again, I don't work in this
> industry.) I had assumed that by now, making something trivial like a
> cube-shaped box would be easy.

Surprisingly something simple like a cube shaped box is problematic for 
a variety of reasons. The sides are large, flat and almost perpendicular 
to base (or at least you want them to be). This makes it much more 
likely the part gets stuck in the tool, parts stuck in tools = $$$. 
Plastic shrinks as it cools, large flat surfaces = twisted and warped 
parts. Usually you would put supporting ribs on the inside, but in a box 
I guess you want to actually use the space inside for storing things, so 
you are a bit limited what you can do.

> That's impressive, given that even "real moulded plastic" isn't strong
> enough to make a gun... They're made out of metal for a reason, after all.

Well it's clearly strong enough to fire at least a few rounds (there's a 
video on youtube I think). And if you're the sort of person who wants to 
try and get a gun through some security checkpoint then you probably 
don't mind that it won't be a long lasting reliable weapon.

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