> The point was that the stated danger of 3D printed guns is their use by criminals for criminal purposes, not economic competition to established gun manufacturers.

I guess the counterpoint is that it's not actually useful to criminals either, so there is no incentive for any non-fool to want laws like this and then all incentive arguments are weak because foolishness can be attributed to anyone.

Let's review the three possibilities here.

One, you succeed in never being identified or apprehended. Consequently you, rather than the police, have the gun you used, and you can file off the serial number and throw it into the sea or whatever. They don't know who you are so they never come looking for the gun you no longer have and it's just one of millions that were sold to random people that year.

Two, you get caught before you do the murder. Some cop thinks you look too nervous or you get into a car accident on the way there etc. and they find the gun. Having one without a serial number at this point means you're in trouble when you otherwise wouldn't be. It's a disadvantage.

Three, they catch you in the act or figure out who you are because your face got caught on camera somewhere after you took off your mask etc. At this point it's extremely likely you're going to jail. This is even more likely if the weapon is still in your possession because then they can do forensics on it, and it not having a serial number at that point is once again even worse for you. This is apparently the one that actually happened.

Whereas the theory for it allowing you to get caught would have to be something like, they don't know who you are but they have a list of people who bought a gun (which, depending on the state, they might not even have) so they can look on it to find you. But that's like half the US population and doesn't really narrow it down at all.

There is no criminal benefit in doing it so that leaves the remaining options which are either trolling or stupidity.

You call these project[s], which I think is very accurate for the higher power cartridges. You sound like you've seen a lot of the videos of 3D printed firearms, and from what I can tell they cluster around 9mm and 5.56. There's probably multiple reasons for that, one of which is that those round sizes are more widely available and cheaper, while another is that it is going to be easier to do than something with higher power. So to maybe simplify my point, the technical challenges and inherent safety issues on 7.62 are higher. Thus, projects they shall remain.

I highly doubt that anyone who 3d prints a lower does so to “use” it (I.e. shoot someone) in order to procure a better firearm.

Sure, I can imagine any number of motives and Rube Goldberg mechanisms for procuring a firearm to service that motive. My point is that if someone who is desperate to get a firearm has to 3D print one they’re going to pick a simple pistol lower. Not something for a rifle that fires a higher power cartridge. Most rifles that fire 7.62 are not in the AR format.