The main reason this allows you to build an AR-style rifle is that US firearms law focuses on receivers (per U.S.C. Section 921(a)(3), which are in many cases quite easy to manufacture. For instance, many Kalashnikov receivers are made of stamped sheet metal.
The pressure-bearing parts, like the barrel or the bolt, are much harder to manufacture, at least for rifles. Most European countries will therefore regulate these, but they won't necessarily regulate ancillary parts like receivers or stocks. AFAIK, a full-auto AR receiver, which is highly regulated in the US, is treated as nothing more than a chunk of metal under UK law (as long as you don't illegally assemble it with a barrel and other parts).
[I was wrong; according to the 9th report of the Firearms Consultative Comity, Annex D, receivers are controlled as "component parts"]
Plus the way the law is structured in the UK, you cannot out-clever it. It is a "If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck."-type of law.
So for example, a farmer (without a firearm licence) tired of people stealing created a trap, a thick cardboard tube, hung on a string pointed at the doorway, set to ignite and shoot shrapnel when the door was opened.
The farmer winds up forgetting about his trap and sets it off injuring himself. He was charged with having an illegal firearm because even though it was something he built himself, it was still similar enough to a gun to be considered one.
> Plus the way the law is structured in the UK, you cannot out-clever it.
Funny, that. The US has a similar on the books with regard to drugs - the Federal Analog Act -http://en.wikipedia.org/wiki/Federal_Analog_Act - if some compound is "chemically similar" to a schedule I or II drug, then it can be treated as if it were also on those schedules.
The pressure-bearing parts, like the barrel or the bolt, are much harder to manufacture, at least for rifles. Most European countries will therefore regulate these, but they won't necessarily regulate ancillary parts like receivers or stocks. AFAIK, a full-auto AR receiver, which is highly regulated in the US, is treated as nothing more than a chunk of metal under UK law (as long as you don't illegally assemble it with a barrel and other parts).
[I was wrong; according to the 9th report of the Firearms Consultative Comity, Annex D, receivers are controlled as "component parts"]