Aren't we moving towards taking people off the battlefield? If you look at what's going on in Iraq you'll see that a majority of the attacks on Iraqi civilians and US personnel are being conducted using bombs, not rifles - which, aside from being sneakily effective, removes insurgent combatants from the battlefield. There are plenty of ambushes going on too but they haven't proven nearly as effective for our enemies as those bombs have.
Russia, 50 years ago, had so large a number of tanks that they had more than us, Britain, and France combined. WWI and WWII were harsh lessons for the Russians, lessons which they learned well enough - they'd have used their tanks on us any way they could. In terms of preventing a successful invasion, no, a group of American citizens with automatic weapons wouldn't have done a whole lot of good. In terms of starting an insurgency afterwards, they'd make a good start - however, with registration laws existing previous to an invasion and occupation, the Russians would have access to comprehensive lists of everyone licensed to own and carry an automatic weapon. The Soviets were hardly the kings of efficiency but I think they'd have managed to see a threat like that, don't you?
And China, besides having the largest standing army in the world, also has the fifth largest nuclear arsenal in the world and are considered by the NPT to be a "Nuclear Weapons State." An all-out war with China probably would never see an invasion of either country; they know that if they had us so far up against the wall that their troops were on our soil than we'd launch nukes at them. They might go ahead and do it anyway, but you can bet that our leaders would hold up their end of the scenario - they'd launch nukes at China, the Chinese would launch them at us...and then it doesn't much matter if you have an AK-47 or a popgun.
I'll concede your point about semiautomatics as you are clearly, to me, correct. However, I do believe that things like armor piercing rounds and automatics shouldn't be part of the US household. Statistics have proven that allowing citizens to carry firearms reduces crime; however, at the same time those statistics were recorded (think mid- to late-90s) there was a ban in effect on automatic weapons in the US (it has since expired and no great drop in crime has been recorded anywhere). If you're facing a robber who is wearing body armor you're facing one rare robber indeed.