So on durability it seems the converse has usually proven out to be the rule rather than the exception. Probably the best example is the AK, tolerances are loose, it can be manufactured with rudimentary equipment. Basically a metal brake, a lathe and a rudimentary milling machine to machine the gas block and trigger group. The entire gun is pinned together and can be taken down in the field with a punch. It really is so simply built to the point of being genius and while it is not as accurate as some of it's counterparts produced during the same period, it is accurate enough. It is generally regarded as the most reliable gun ever produced.

The converse story of that would be that of the AR-15/M-16 tight tolerances and precise engineering lead to it fouling in action due to dust and debris to the extent that it went thru several decades of revision until we ended up with the modern AR-15/M-4.

As far as barrels I would say that is the biggest issue with ever production a 100% 3D printed gun, unless there are serious advances in modern plastics that lead to an extremely ridged, heat resistant, and expansion resistant material they will continue to need to be metals with high tensile strength. It needs to be rigid to resist barrel wrap where the barrel flexes and reduces accuracy under fire. It needs to be heat resistant for obvious reasons gun barrels get hot and it needs to resist expansion so that the barrel is in contact with the bullet at it travels down it. Thus metal seems to be the only possible material at the moment that contains the requisite properties for barrel development.

Beyond that, even if they can get to 100% of all other parts being manufactured and using common pipe with some chemical treatment to overcome the barrel issue there are still some major disadvantages that cannot be overcome without at least rudimentary metal shop tools. The biggest being that just using a pipe gives you the equivalent of a smooth bore slug gun. For any amount of accuracy the firearm has to have riffling which is groves cut into the inside of the barrel to cause the bullet to twist as it moves down the barrel thus stabilizing the flight path of the bullet when it leaves the firearms barrel. I just don't see how the above issues can be overcome via 3D printing. That being said, it's actually pretty easy to home manufacture a firearm with the most basic of metal shop tools.