Evolution of CNC was for reference of technical improvement over time. I would expect similar improvement for this technology over 50 years. Especially because there is already commercial applications for it. The other key part of this technology is the HUGE drop in skill needed to make it once someone has worked out the design. The material science issues will be tough. But material science evolution has not slowed down over the past decades. It just does not get big press like computers.
Now that I think about it. If barrels are the only issue, improvements in other machining technology could take care of te rest if the equation. Also, is swapping a barrel from a legal weapon a possibility. For instance a legal high pressure single shot rifle. Strip it's barrel download a design and you are done.
The way these things are now the best you could really do is use a very nice prototype to produce investment casting molds. Of course, one would have to already be a gun maker to properly cast the parts and machine them into anything worth a crap.
So yeah. Cool trick, but like you said, absolutely pointless for production at this stage.
I can see CNC technology advancing to the point where all this is possible. User friendly software and soforth. But some things can't be overcome. First of all - by nature, a lot of this machinery has to be sturdy and rigid. Heavy is good. Parts need to be cooled during machining. Lots of tooling is required for machine work with multiple operations. This tooling wears out. Users have to be knolwedgeable enough to replace it. You're dealing with a big, heavy, oily hunk of machinery that most people will never have a place for in their home. And there will always be a certain level of knowledge required. Maybe it could all become easy and affordable enough for savvy do-it-yourselfers to have in their garage. I don't see it ever becoming easy or practical enough for everyone though.
A note on the barrels: a genius will figure out a design based on a common legal barrel. Joe Six Pack just downloads the design. I don't know enough about machining to go deep into the technicals. But in most engineering/build applications the engineering is never enough. Skilled craftsmen have to do he build. Now unskilled craftsmen can build a design. This is the transformative part of this technology.
When they can print working bullets, then I'll be impressed.
There's really no such thing as a common barrel. Barrel blanks come unthreaded with no chamber. You cut the threads to match the receiver and the chamber to match the cartridge. Some receiver/barrels aren't threaded, but it doesn't matter. If your plan is to scavenge barrels to put on this imaginary receiver, you're going to have to take what you can get. That means remachining the barrel to match the receiver.
If you have all these barrels to scavenge, why would you not just use the gun you're taking them off instead? If you're scavenging, you're way more likely to come across perfectly good semiautomatic weapons than single shots. You'd be downgrading to take off the barrel and put it on your homemade action.
Designing an action around a barrel is ass backwards. That's not how it works. Cartridge first, then action, then barrel.
Sorry, not trying to move goal posts, lots of speculating on my part and just started researching this. Obviously the technology already affects how AR 15 enforcement (the law remains the same) because they have a working receiver design already for 3d printers.
I am not convinced the material science issues of making a barrel won't be overcome. Just have to wait and see.
As for using a barrel from a different weapon I do think you are right about the obstacles. However, the 3d printer may allow some sort of conversion kit or selling barrels without the rest of the weapon. (I know moving goal post again, but this is mostly speculating as I post)