The Wiki Weapon is real, and it’s centerfire. Am a bit surprised to hear that. The only non-3D printable parts are the firing pin, a slug to make the gun compliant with the Undetectable Firearms Act and of course, the ammo.
It’s a single-shot pistol and they’ve developed it to have interchangeable and replaceable barrels. It’s pretty clear that you can overdo it caliber-wise. I’d stick to the low-pressure stuff myself, .32 ACP, .38 S&W, .38 SPL.
There’s still a lot to be done here, but congratulations are in order. If you want to find out more about the 3D-printed Liberator (not to be confused with the stamped steel Liberator) check out this interesting write-up on Engadget and the project blog.