I've been in the US for nearly a year now, and wouldn't put much faith in some of the lard-arses i've seen carrying guns here. Yes, some ex-military types who you could trust, but the local civvy populations of places, where i've been staying, I don't think so.
I like the US right to bear arms, it is good for the US, maybe not other countries. Its a good second line of defence (after the army). However, Militarily, I think it would only provide a token resistance. It was Ok 200 years ago, against army's who used the same weapons, but not against todays high tech army's.
<sighs> Another European who doesn't get it. But that's okay.
The military function of the militia in U.S. history has been a) protection against Indians and other groups of predators, and b) against a force of regulars, to deny them the use of the countryside and easy access to people and resources, and to confront only if necessary or advisable.
An example of the latter is the campaign of General Burgoyne through upstate New York, who descended from Canada along what turned out to be an arduous trail that taxed his engineers, but which they nevertheless succeeded in cutting down to the valley of the Hudson. They were supposed to descend the Hudson and effect a junction with British forces then quartering (another thing we don't allow any more) in New York. The militia opposed Burgoyne much of the way, harrying him continually, and standing up to him at Bennington. They lost at Bennington, one of the biggest battles of the American Revolution, but their objective wasn't necessarily to win, but to impose a Pyrrhic victory. It had the desired effect, and after meeting the militia again with the American regulars at Oriskany, Burgoyne was undone, the Americans having meanwhile prevented his communicating with New York.
The function of the militia is not to fight and win a stand-up fight with an enemy formation. Short of that, it can do a lot. Rudyard Kipling seemed to think pretty well of our system: he visited the country in the 1890's, and formed the opinion that a Great Powers expedition against the continent would have only one outcome: "They would die." He got it right.