There are legitimate uses for firearms that are neither bad nor nefarious.
true - but not for an inaccurate easily concealed HANDGUN that fires 22 long rounds.

Guns are useful tools - handguns - not so much. Plastic untraceable hand guns - yeah, only two uses really.
1) plinking in your back yard.
2) criminal purposes.

Given that handguns - in general - are illegal in most countries anyway (america is JUST 1 COUNTRY - no matter how many times you slap the word 'world' in front of a purely american event, it doesn't make it so)
What it could be used for is a moot point. Just having the .stl files on a computer would be enough to be prosecuted in the uk.
And much as I'd like to make one, just for the sake of it. I'm not stupid enough to think the .stl downloads aren't being tracked by GCHQ.