Composite Pistol in "In the Line of Fire"
 

Does anyone know exactly why it had to be destroyed after filming?

Legal issues, apparently.

What those legal issues were, is what 2wingo's question is. And one I kind of share.

I imagine there are two parts to this. Firstly there is the "Undetectable Firearms Act of 1988" which would make possession of the pistol illegal in the US (building such a pistol is illegal as well, I'm assuming the production either got a special exemption or it was exempt due to it being for theatrical usage). Secondly the gun was sold as a movie prop (I believe the picture is from the auction listing) rather than as a functional firearm (which would have been much more legally complicated) so it had to be deactivated first. When the pistol was sold I believe it had velcro pads so that it could be stuck back together in the shape of the original gun.

Oh, I'd never considered before that the thing actually worked, I just thought it was a full scale plastic model of a gun designed by a five year old.
I suppose making it fire blanks was the easiest option, seeing that the movie predated Adobe Muzzle Flash PRO.

The other thing with the gun which may have made it legally "tricky" is that as it is a simple break open single action gun, if it was capable of firing blanks then it probably would have been capable of firing live rounds. The gun might have destroyed itself after the first shot, but it would have worked.

I remember that. It came from the Bullshit urban legend that the GLOCK was invisible to Airport metal detectors, remember those days? the gun control idiots got a lot of mileage from that one, even though NOW, most police departments use the Glock.

It expired in 1998 but was re-authorized in 2003 and is set to expire this december (2013). With the advent of 3-D printing, you KNOW, some idiot politician is going to make a law banning the mythological all plastic gun again..... :D

What exactly IS 3D-printing, vis-a-vis guns, and why is everyone in an uproar about it?

A 3-D printer can basically create anything out of plastic, including high capacity magazines and gun parts. This would render many gun control measures moot.

That doesn't seem like it could make a very GOOD gun. I mean, there are certain things you just can't do with plastic. Even Glocks have a bunch of metal parts in them.

Tell that to the gun control people. All they see is OMG PEOPLE MAKING FULL AUTO ULTRA DEATH MACHINES THAT CAN'T BE SEEN BY METAL DETECTORS.

Yeah, a bunch of folks on cracked thought the only reason for a poorly made disposable gun was for nefarious purposes.

I thought the issue with the plastic pistol was that since it was capable of firing, and up for sale by the production company, that it had to be rendered inop due to lack of an -07 FFL.

Even if you can't see the gun, you can see the bullets.

Hell, metal detectors can detect a belt buckle or small pocket knife. I'm willing to bet that even a single cartridge would set it off, especially one of the handheld wands.

Also, the plastic in firearms is more than dense enough to show up on standard airport x-ray scanners.

As for the 3D guns thing, the guns are indeed made mostly of plastic. However, you still need metal for the important bits (especially a barrel, and if you want accuracy you need to be able to rifle it) and anything more than .22LR will probably cause something to break off within a few shots. Someone did make a 5.56mm gun from 3D printed parts and it malfunctioned several times and fired something like 3 rounds before the stock came off.

The biggest uproar is probably going to be internet censorship and 3D printer regulation rather than gun bans. One of the biggest concerns people have is that the ability to print "high capacity magazine clips" and "assault weapon parts" from your own home with the right materials and a few thousand dollars of equipment will make firearms bans nigh-worthless, and will inspire legislators to instead attempt to regulate 3D printing technology in general and censor internet distribution of firearms plans.

Of course, at least one website put out a free torrent of all of their 3D printer blueprints for firearms parts as soon as people started talking just to stick it to them.

In short, gun control measures are stupid and don't work.