Further plenty of semi-auto guns (ar-15, mini-14 are very popular for this) are easy to convert to full auto.
Or, in a pinch, be good with a freaking paperclip.
Follow up WRT to the MAC-10, as that's the most prominent case where the ATF shut down production of open bolt versions for the "civilian" market (civilians can buy machine guns if made before 1986, there are about 100,000 of them in legal circulation).
Since it's fairly easy to make an automatic weapon given a barrel (the metal working isn't that difficult) ....
Yes, there are lots of "plans" circulating, but that doesn't prove that they work. Selling bogus plans is the perfect crime because the buyer can't complain.
FWIW, I've had a standing offer for years to anyone to demonstrate a conversion and have yet to have anyone even try. (I'll pay reasonable costs plus $5k AFTER the demo. Yes, you have to front your expenses - I'm not paying for failures or fraud.)
Since I'm not a LEO, it can't be entrapment.
> I've personally seen a few conversions (in a different country, but same principles) so I know they exist and can be done.
I've no doubt that readily convertible guns exist. My point is that they're (and the relevant parts) are regulated as machine guns in the US.
The "principle" is that the receiver doesn't accept the parts or the parts, and possibly the receiver, are regulated as machine guns. For example, M-16 parts don't fit the semi-auto AR-15 receiver.
I have no idea how such guns and parts are regulated in other countries, but wouldn't be surprised if the laws were different elsewhere.
After all, some other countries treat silencers as good manners they're strictly controlled in the US. (Yes, I know about the plastic bottle trick. It's hard to ban duct tape and a plastic bottle, but it is a single shot device.)