He did. Six shots is merely an unreliable gun, not, fundamentally, a non-functional gun (see also the point about 6-vs-100 shots and 0-vs-1 shot), and frankly, an AR-15 seems unnecessarily ambitious for a proof-of-concept. They probably would have gotten better results with something better suited for printing; I bet AK's print better. AFAICT, it's just a matter of sufficient precision with sufficiently strong materials.
One of the major arguments in the article is that instructions exist for printing an Assault Rifle (AR). I doubt that the rounds fired have the same lethal capacity of a "real" firearm; however, I certainly wouldn't want to be hit by the weapon in the video.
The "AR" in "AR-15" stands for "ArmaLite", the original manufacturer, not for "Assault Rifle". An AR-15 is not actually an assault rifle, which implies the capability of fully automatic fire (pull the trigger, spray bullets). The AR-15 is semi-automatic (one shot per trigger pull, but with no manual action required to reload).
Here are a few simple methods to make working guns with 3D printers.
1. Print plastic shaped like gun
2. Load bullet.
3. Fire bullet with now destroyed gun.
1. Model gun
2. Print wax cast
3. Make cast
4. Melt and pour steel into cast.
1. Buy everything except a upper receiver without a gun permit.
2. 3D print an upper receiver and attach to normal gun parts.
3. Use like normal gun (for a little bit)
* «MPEG 3»: Yeah, sure.
* «We can print working guns right now»: No, you cannot, neither you will be able in the foreseeable. Please prove otherwise.