I have to think the easiest way to smuggle *real* assault weapons into the USA would be to conceal them inside bales of cocaine, because nobody ever thinks to look there.
Or you could make them *legally* in your own shop. I made a Sterling submachinegun about 2 years ago, a classic British weapon from WWII. It took me a good part of a weekend, because it was the first one I'd made, and I was making it to be semi-automatic and fire from a closed bolt, not selective-fire, which took a little more work. If I were not interested in complying with the law, it would have been simpler by far to make the more historically-accurate *real* submachinegun. And I bet I could set up a shop to churn these puppies out for a production cost of < $150 or so.
If I were An Evil Criminal Type, I'd surely have some of my minions doing something similar, except for the fact that it is nearly impossible to compete with the former Soviet-bloc nations on production costs, and they have warehouses full of *real* AK47s and AK74s to unload. They'd probably throw in a crate of RPGs if you buy in volume. I can never find RPGs at gun shows for some reason...