You can't print a working spring. You can't print steel. You can't print anything with moving parts unless you print the parts separately.
In addition to the zig-zag style spring as posted above with that magazine, this video shows a more conventional spiral spring.
Obviously nowhere near the strength of a metal spring, but it works for some applications.
You can actually print in steel btw. there are a couple different methods. one requires the part to be hardened in a kiln after printing, the other doesn't. I've had stuff printed in the stuff that has to be hardened before and there is slight shrinking and warping that can go on if your part is thin, so steel printing is not a good solution for precision parts, at least with the technology that I've used.
Plastic printing on the other hand is more than capable of printing more precise parts, including fully captured moving parts like hinges and such which are printed in one piece and work directly out of the printer. That rotary engine posted above is probably the best example you're going to find.
I agree with the poster above though who stated that for the near future we're only going to be seeing gun accessories being made with 3D printers, as practical gun parts are still best made by traditional means. I personally have my designs printed through a website printing service that uses professional machines, because of the higher print quality and variety of available materials. Also, home printers are not as easy as "plug it in and start printing". There is a lot of calibration and troubleshooting involved to make things run smoothly. Most of the people that I know who own a desktop 3D printer are not currently using them because they aren't working.
A set of custom grips for my Ruger Mark III are on my "to design" list. I have made a few sets of custom swiss army knife scales that came out really well.
*first post btw. hi all*