Quite simply, a 3D printed firearm would be the most circuitous and difficult path to obtaining a firearm in the US, whether acquiring a firearm legally or illegally.
On one hand - but on the other it would make anyone with a 3d printer capable of producing an untraceable and fully disposable firearm. For a few bucks.
And that's not a good thing.

Using the same argument that scientists have used down the years: ie: 'we designeda gun but we don;t want it to ever be used against people' - also does not work. have you met 'people' ?
Give them the means to make a cheap firearm - and they'll do it and people will definitely get shot by it.

You designed a GUN because it was a challenge.
Why not design any number of other things that are a challenge, that can't be used for hurting someone else.

Nope - that just doesn't wash. The reason for designing a gun is 3 fold.
1) huge publicity, I mean massive world wide publicity.
2) you like guns - that's not a crime, but it's rarely given as an honest answer.
3) you're some kind of fanatic who believes that the only way to make the world safe is to give everyone a loaded gun.

There is no moral high ground for you on this issue WarFairy.

I can see the attraction of desiging a cheap gun. But I'm also not blinkered or self delusional enough to refuse to see how it will be used and by who.

And yes, you can say if it's not you designing it - then it would be someone else. Alas this is also true.
That doesn't make it right.

And using .22 long ammunition is the right move for a gun designer but the wrong move from the point of view of an englishmen. As it's the most widely used ammo in my country and the one it would be easiest to get without a licence.
At the moment in the uk the criminal element doesn't often use firearms.
So thanks for trying your best to change that.
I'm sure all the future victims of plastic guns will also thank you.

A gun is NOT just an engineering and design challenge - never has been, never will be.
Own up to that or quit.