There actually are some really cool proof of concepts where people have 100% 3d printed a gun. And “potato cannons” are generally a PVC firing chamber and barrel.

For legal purposes: Preface everything after this with a dozen "allegedly"s. A couple years back I went to a really cool event where people had built machine guns out of wood and plastic (FDM). No metal. Everyone went old school with leaf springs and the like. None had any issue with doing well over 100 rounds (5.56 or 5.45) each. That said, anyone with half a brain cell was literally hiding in a bunker nearby out of terror when the firing was going on. But… yeah. Also for legal purposes: I am sure every weapon was registered with the ATF prior to the event and completely destroyed on day two.

Like anything, it is about pressure, strength, and geometry. And, as The Troubles in Ireland can attest… you don’t need THAT much skill to make something that will fire… once. Which is why there are so many shed machine guns and rifles with split barrels and completely exploded receivers in The Royal Armouries.

But if you want to make a barrel that can hold up to even a few dozen rounds? You need to understand what metal to use (hint: it isn’t mild steel) and some pretty decent lathe skills.