Just my $0.02, but I don't think polymer FDM technology will ever be suitable for gas system components. So if you go down that path, you're looking at sophisticated DIY metal parts that it will put the design out of reach of most people. ECM may be viable for some of these parts, but there's a pretty big skill gap between cutting a barrel tube to length (where actual length doesn't matter at all to the process), and cutting bar stock to the rough sizes and shapes needed to fit ECM jigs. Take a look at the Rogue/Rebel 9mm bolt instructions for an example of a steel assembly that's pretty close to the limit of DIY-ability.

IMHO, the key to handling more powerful cartridges is figuring out how to do a delayed blowback system that is easy to DIY. I'd suggest doing some research into lever-delayed systems (Forgotten Weapons has a good intro here: https://youtu.be/utv6LK6CxJI ) and thinking about how existing DIY bolt designs might be adapted to utilize that. I don't know if it's practical to get all the way to full-power rifle cartridges with a lever-delay system, but it definitely has potential for intermediate cartridges like 5.56x45 or 7.62x39.

I would suspect the right way to work our way towards a fully DIY intermediate caliber design is to handle one part at a time. So start with a design that only DIYs the barrel, or the bolt, etc., then progressively step towards turning more parts from commercial to DIY.

I'd be curious to hear more about your ECM taper idea. What can you share about that?