The US' crime problems don't have much to do with immigration.
And the European economic problems stem from something much different, mostly aftershocks of the 2008 crisis and the idiotic austerity measures introduced as a result of that (which are exactly the opposite of what I'm arguing in favor of), as well as some fundamental problems within the EU mostly having to do with trade balances between individual countries, that didn't really cause much harm as long as everything was going well in the period of 2001-2008, but are now showing.
To your first parts, "criminals" aren't a seperate species, they're simply humans, and even though they're breaking some laws doesn't imply that they'll out of some reason break every law there is, which seems to be your underlying argument. In any case, nationwide restrictions on guns that are actually enforced would make it much harde (not impossible) to obtain guns, which would do a lot of good already.
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Also it's just a city-wide gun, which is much much easier to circumvent than a nation-wide one would.