First, Schatten, a caveat: I'm not saying they would
win. I meant "credible" as in "would be more than a nuisance"; I was perhaps not clear on that.
Two problems: One, the largest caliber you'll probably find in any major quantity is going to be the .50 BMG, which is going to do very little against a Stryker or Bradley. Two, the majority of these idiots have very little experience with actual assault rifles, instead bolt-action which unless they decide to be ignorant and try and fight a pitched battle is going to have very limited use in an urban environment.
This is where my ignorance of gun ownership comes into play. I was under the impression it was fairly simple to modify a hunting rifle like the AR-15 to make it essentially a full-grade military assault rifle; I furthermore have been inundated with youtube videos of what's colloquially known as 'gun nuts' gleefully firing semiautomatics, which probably isn't at all a representative sample. So I'll defer to you on that one if I must.
modern infrastructure and technology that is trivial to procure
Like what? I mean yes, you do have a lot of items that can be used for making things like IEDs, but unless it's one of the really crazy survivalist nuts, they aren't going to have knowledge of making explosives.
You seem to approach this from a strangely small-scale perspective. An effective insurgency or enemy force would be large, and organised, to present any sort of threat; it wouldn't be down to individuals of exceptional skill, but to the total pool of people. According to the OP, we were talking about a not-insignificant part of the
country.
Given that, in response to your question, things get a little more interesting, because nearly everything that the civilian world has to offer would be possible to procure given large enough numbers of "everymen." High-grade electronics, chemicals, weaponry.
Again, the education American's recieve is for making them effective members of society, very few turn out to be gunsmiths or chemists of any type.
An insurgency does not need a surfeit of gunsmiths or chemists. What it would need is a broad range of skills and knowledge of diverse topics. Or, in other words, education.
How would building up for the insurgency help? American's don't have access to unexploded 155mm and mortar shells so no real big IED's and the best you can do is stock up on bulk orders of .308 Winchester or pipe bombs.
I'm sure Timothy McVeigh must have felt much the same way.