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.