Here in the UK, just being in possession of a home-made handgun is going to get you a hefty (mandatory minimum: 5 years) prison sentence.
Being in possession of part of a rifle or shotgun might be legal, depending on whether you have a firearms license for that particular device. But I'm pretty sure AR-15s are blanket banned (semi-auto rifles have been illegal since the aftermath of the Hungerford massacre).
Now.
Is it legal (or not) to merely download the 3D model over the internet in the UK? What if you, for example, printed it out ... but after applying a distortion that renders it unfit for use as the receiver of an AR-15? What if you've scaled it up by a factor of two, so it could in principle be the receiver of a 1:2 scale (large) copy of an AR-15?
(What if, in the USA, you download the file coding for the magazine and expand it to hold 30 rounds?)
Edge conditions: chewy!