So far, the eggheads pioneering the open source 3D printed guns seemed focused on replicating an AR-15. But eventually they'll get over themselves and start picking at the low hanging fruit of that tree.
The gun in "In The Line of Fire" is a perfect example of what can easily be done with 3D printers today.
The gun from the movie was a plastic double barrel 9mm, most likely, and it walked past metal detectors with ease. Because 'Booth' only intended to fire two shots from the gun, it didn't need to be built to the high demands of an AR-15. And for self-defense purposes most people who would like to have a gun won't be firing hundreds of rounds through it. They want it for peace of mind, and want to have it around 'just in case'. Lots of gun owners would be fine with just such a disposable gun that could only be fired a few times before it destroyed itself.
And most fire fights happen within 16', even a sloppy, inaccurate gun like this would do the job just fine (if it had, say 25 rounds instead).
Enter 3D printing.
For a movie and a skilled model maker, two barrels would be difficult enough to get to work reliably.
But for a 3D printer, making 25 barrels and 25 firing pins would be simple. And because it lacks a removable clip, it would be exempt from the current gun bans as written. 25 rounds of 9mm would make a barrel 3"x3"x6" long. The trigger could be as simple as pulling a blocking strip from between the pin and the back of the bullet. As strips, it could be fired as fast as an automatic with a quarter a trigger pull being geared to firing a quarter of the bullets.
Of course, disposable guns that can be burned in a fire after use and only cost a few dollars in plastic and an hour or two of assembly would have criminal applications too.
But ponder this, with 3D printing, guns like this can be made to look and feel like toys, even super soakers or ray guns, making them difficult to confiscate if homeland security starts going door to door to confiscate your 'real' guns. Why else has Homeland Security bought enough bullets to fight the Iraq war for 25 years, if not to disarm law biding citizens?