The problem is accuracy. Chamber specs are a lot more complicated than people realize. In many cases, walls are tapered so you're working in all three dimensions at the same time (eg. changing diameter also changes depth) and you have to control multiple diameters with differing wall tapers simultaneously. Typical SAAMI chamber tolerance specs are +0.002" (+0.05mm) on diameters which is not easy to achieve across all dimensions of a printed part. I won't say it can't be done, but the skill, patience, and luck required would be off-the-charts. Realistically, if someone can get the ammo, they can probably get their hands on machined parts, or at least the machine tooling to make them with a lot less time and effort.