The difference is that No technical engineering skill is required to make a gun via 3d printing.
And the equipment to make a plastic gun from someone elses designs cost as little as $250 for a kit and $400 for a ready built machine.

THAT'S the difference. 3d printing puts the ability to make complex objects in the hands of people who lack the skill and education to make them any other way.
And the printers are now cheap enough for anyone to have one.

So people who wouldn't have considered making their own zip gun. Might happily print a 3d gun for a couple of dollars and a few hours print time.
No skiill required.

The whole lathe and engineering argument is spurious as you are not comparing like with like.

Okay printing metal gun components will cost you a lot more in initial investment. But once you've installed a metal printer you can print full spec firearms - again with no technical knowlege needed other than how to run the printer and put the components together properly.

It's actually not as big an issue as people supposer. In the usa - where guns already outnumber people - why bother printing what you can already buy in a general store.

In the uk printing a gun is somewhat pointless as you need a firearms licence to buy ammunition. So you'd have to obtain the ammunition liiegally - at which point you'd probably buy an illegal gun.

It will be an issue - but not as big an issue as most people think.

Also I'm pretty sure that all the known repositories of plastic gun files are most likely being monitored by gchq. And anyone in the uk who downloads such files will probably get a swift visit from some fairly unpleasant gentlemen.

Someone else can test this theory - I'm not going to :-)