The UK law is pretty simple...
All the legislation is laid out on pretty easy to read language. (that should be the most up to date with all amendments etc)
http://www.legislation.gov.uk/ukpga/1968/27
(just search for the term manufacture)
We don't have a constitution of rights. that muddies up any points or anything like that.
for the purpose of UK law we'll deal with Shot guns, Hand guns, automatic weapons and then Firearms.
Shot guns,
Any body can "use" a shot gun. -that is to say you can go shooting with a friend.
anything more than borrowing a mates gun requires you to apply for a "shot gun certificate" it's a reasonably easy to get certificate that allows you to own guns, (as many as you like), each gun must have a serial number and must be kept in a secure cabinet (where there are laws surrounding the gun cabinet constructions and fixing to the wall).
it's illegal to have a gun in a public place (where a public place includes your car). without a good reason - e.g on the way to/from a hunt/shooting session. -so you can carry your guns out your house, walk down the road to a field, do some shooting etc, then walk home, but taking your guns to the pub en-route whilst walking would be foul of the law.
same applies to a car, but to a greater or lesser extend anyone who's ever been on a hunt knows that everyone goes to the pub afterwards.
Hand guns.
You can't have them, they've been outlawed for well over a decade now, since the dunblane masaccre. (where a scoutmaster murdered a class full of five of six year old kids plus teachers.
Automatic weapons.
again restricted by law, -they are explicit exceptions to what is classed as a firearm in law -that is described in the act linked above, automatic weapons are not firearms in the UK.
Firearms.
require a different certificate than you get for keeping shotguns, (think of the shotgun cert as the easy to get cert -requiring only respected members of the community to "vouch" for you). a firearm cert takes more paperwork, and more specific recommendation, -and they are renewed more frequently. firearm law coveres everything that's not a hand gun, shot gun air gun or automatic/semi automatic weapon. (basically riffles).
Airguns,
pretty much a free for all. - as you may have guessed from my talk of hunting etc I've in the country rather than the city, it's common place for people to own guns, I have air riffles when I was growing up. and have been clay shooting, know many people who still clay shoot, and have been hunting for food, as well as having seen organised game shoots...
however, if you live in a town, I would suggest not performing target practice from an upstairs window into your garden, when a neighbour calls the police, they can and will respond with an armed response unit, who will have you on your knees with hands behind your head whilst they search your house. (this actually happened to a friend while at university) -actually it more happened to his house mate, in the middle of a romantic dinner.
For the manufacture of firearms.
clearly, once finished making a firearm you'll own a firearm -so much be licensed to do that.
any gun barrel produced for sale in the UK must be proofed.
http://www.gunproof.com/Proofing/proofing.html
gun makers are still proofing guns.
http://www.gunmakers.org.uk/proofhouse.html
I haven't investigated their site too far as I don't actually want to make a gun. but it looks like if you join them then you can design and make guns, have them proofed, serialised recorded and ready for sale as a pretty easy process...
In the UK -just like in the US and anywhere else in the world, if you want to make a gun then there is very little technically or legally to stop you. get the right permit/certificate to own the gun you're going to make, get help (there are a number of gun smiting forums who will point you in the right direction). and it wouldn't hurt to go and let the local fuzz know what you're getting up to in your garage. -not a requirement but a really good idea. and will go a long way to help your case when you're raided and 3d printer parts are thought to be guns...
as said I've go no interest in making a gun...
however, it is something I've looked at before...
If I were to make a gun...
It'd be a traditionally made, (and lovingly hand crafted) shotgun, by traditionally made I mean using traditional method, smoke fit etc, (smoke fit is a method of creating perfect clearances using soot from a candle to blacken a surface, -you then assemble the mechanism, then take it apart, any bit where the soot is worn off clearly is too tight and must be adjusted.)
The barrel would certainly be proofed, not because I'd intend to sell the gun (then it's be a legal requirement), but because it's one of those really good idea type deals, one of the big reasons that Britain managed (hundreds of years ago) to murder half the world and claim it's land as our own is that we tested our gun barrels, others didn't and they had exceptionally high fatality rates related to barrel explosions.
The only way a 3d printer would be involved is in either making tooling, -to aid the manufacture.
or making prototype parts (e.g testing a lock mechanism)
or making parts for a lost wax type casting (except lost PLA).
a final note about guns in the UK.
even plastic replica and toy guns that look real will be considered dangerous from a distance, the police will shoot first and ask questions later -that's why toy guns have that big orange plastic cap on them.
ronsii wrote:Hi Antron007 
I pretty much agree with you and would rather make em' out of metal 
but... it is always cool to apply new tech to old processes.
Oh, depending on where you live in the US. handle mods can be illegal.... why? because it make the firearm so much more scary
the public have a right to feel safe in public.
That's why you'll be shot for having a really realistic looking toy gun and waving it about.
and that's the thing that I find incredibly annoying about the whole defence distributed guys.
they claim to be all about guns, and free ownership of guns.
but what have they done?
made a few nick knacks on a really expensive printer (though nothing that needs any real strength.
well whoopee, for most of the parts that they made you could carve them from wood, or cast them in concrete.
there have been part finished (and not serialised) lower receiver parts available for decades (that you just need to file to fit)
so it's not even like they made it so very much easier to make a hard to make part...
so having established that they haven't really helped in gun manufacture at all, what have they really done...
They've made plastic guns scary, thus ruining games of cowboys and Indians/cops and robbers for five year old all over the world.
Just had to point out probable juxtaposition regarding 'laws' that will eventuate