3D printed guns right now aren't comparable to hundreds of years of gunsmith engineering. So 3D gun plans & printing really have no significant effect on anything.
-Renegade
Really? A cheap untraceable gun suitable for use at close range doesn't inspire all sorts of creative thinking?
It doesn't even need to be durable. All it needs to do is be able to fire a single shot (or six) with a fair degree of reliability and voila - Disposable guns! One step better than the so-called 'Saturday Night Special.' Doesn't even need to be that accurate as long as it keeps fairly close to the direction it's pointed in. Look at the flare guns found on boats. Most are now made of plastic. And they perform for their intended use as well as the old-fashioned durable variety.
So much for even having (highly flawed) ballistic or manufacturer's data to fall back on for traceability when a weapon has no pedigree whatsoever. Especially if it's ground up and recycled (or even melted a bit and tossed) shortly after it's been used. It's almost like saying: "Imagine a gun."

-40hz
There is a certain appeal to a single shot firearm, but really... few people have the skills to use a weapon like that effectively.
I'm very capable with firearms -- but I wouldn't want to RELY on a single shot gun.
As for cost, I'm not really sure. I don't know the cost of a gun on the black market, so can't comment there.
But I am very curious. I thought that materials were around $5 per cubic inch, but... dunno.
Is the cost of printing a gun lower than the cost of buying one?
As for creative uses... well, yeah... I can think of a few. They all involve close range though.
The average person never has any need to use one, so I don't think it's really much of a worry. Professional assassins will already know enough to get around evidence problems, so they're not a real concern as far as I can see.
But the *cool* factor is definitely there~!
Like, who wouldn't want a 3D printed gun as an ornament? Well, some people sure, but it's still a cool conversation piece!