Meth is illegal, full stop. Guns are not, that "legal loophole" is referred to by others as the Constitution, which makes gun manufacturing subject to laws and regulations, but not bans in the US.
I don't know if this is actually true. The right to keep and bear arms is guaranteed by the second amendment, but its a bit of a stretch to say that includes manufacturing of arms.
Nope, that's not how it works.
I'm not aware of a 2A case on point regarding firearm manufacturing (others may know more), perhaps because the federal Protection of Lawful Commerce in Arms Act offered sufficient protection. I am aware of a case on point regarding selling firearms. Earlier this year, Chicago's ban on selling firearms was ruled unconstitutional: http://www.volokh.com/2014/01/06/firearm-may-sold-acquired-o...
This makes sense if you think about it. The First Amendment right to freedom of the press wouldn't mean much if the Feds can levy a 10,000% tax on newsprint. Which is why the Supreme Court has held that such taxes violate the First Amendment: http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&vo...