I can't remember exactly, but in California you can make your own, but it has to have a serial number. I can't remember what other restrictions they have.
A man in Queensland was given a suspended sentence for printing parts of a gun that did not together constitute a usable gun without some additional pieces[1].
Apparently there have been efforts there to ban 3-D printing of guns but so far they have not been successful.
In the U.S., while it is physically possible to do anything within the privacy of one's home, it is technically illegal to print a gun if you are not legally permitted to possess one.[2]
There are also existing laws on the books against plastic/undetectable guns which presumably would apply as well to resin/PETG/PLA printed firearms that have few or zero metal parts. Actually, one wonders why there haven't been more incidents of plastic guns smuggled onto aircraft in recent years, perhaps because K9 teams of bomb sniffing hounds are randomly used to detect bomb-making ingredients while standing in line at security. They are trained presumably to smell gunpowder as well as other chemicals. Leave your "Gunpowder Solid Men's Cologne" at home.
1. http://www.australasianscience.com.au/article/science-and-te...
The root cause of this is that very few people want to smuggle guns onto aircraft. I think the biggest group of gun smugglers at this point are TSA penetration testers.
For the most part it's always been legal to build a firearm for your own use (not to sell - that would be regulated under the Fed's commerce powers). Some states make it more burdensome than others. Making a high quality anything requires skill, but guns are very primitive mechanical devices, so the bar is very low to make something of effective working order. You won't be hitting a gong at 1000 yards, but you'll hit a pie plate at 20 pretty easy.
Depends on what you're building. If you're making your own AR-15 lower, there's no detrimental impact on accuracy - all of those parts are contained within the 'upper', which (in the US) is not regulated as a firearm.
If you're building from scratch, though, accuracy is really not going to happen. Not unless you have actual machine tools and training.
YouTube censored the vids of casting your own AR lower out of beer cans, but it’s still online[0].
If you have a modicum of skill in the garage you can also fashion an AK receiver out of a shovel (or so I am told).[1]
“So, there you have it. In 2012, a drunk guy named “Boris” -who lived in what he refers to as “Communist Massachusetts”- built an AKM from an old $2 field shovel and a bunch of parts kits that can be found all over the country. His total cost was $232.”
[0] https://forum.full30.com/t/ar15-made-from-beer-cans-complete...
[1] https://warisboring.com/extreme-diy-man-makes-akm-from-a-sho...