I don't get the whole argument of "having guns will prevent tyrannical regimes." The government has tanks, planes, missiles, mortars, body armor, tactics, training, etc. The people have a few assault rifles.
Take Syria. The rebels basically have AKs and some homemade grenades and mortars and the only reason they're winning is because they are 85%+ of the country's population.
I sympathize with this argument but feel like it's off base a bit when it comes to the US military. It is illegal for a member of the military to follow an illegal order (like killing an innocent civilian). Even in Iraq/Afghanistan soldiers have to ask permission before engaging with an enemy depending on the circumstances. The US military is mostly made up of minorities/lower income people from working class families and know-nothing patriot anti-liberal crowd. The patriots may do as they're told but the working class soldiers would definitely not fire on their own people, the majority of soldiers wouldn't. It's always the local police that do the oppressors bidding.
The government would have to already create a situation where an opposing political group are formally recognized as terrorists, 'therefore killing them is justifiable' and so on before any sort of onslaught were to happen and by the time that occurred the amount of civil unrest would likely be at a dramatically high percentage and at least be able to approach that 85% range much quicker. That population already having weapons would dramatically increase their chances for overturning a tyrannical government.
However it's also reasonable to assume that even if all guns were taken, the members of the military who oppose their orders would likely smuggle weapons to rebels anyway, so they wouldn't be entirely hopeless. But basically this debate comes down to whether or not you think there can be widespread public support for some sort of revolution in the future or if it's possible for all changes to be through legislation/within the current political system.
Most people aren't into radical ideas but if they were they'd likely want to do it via the 3rd option which is peaceful mass strikes/protests like arab spring since it works better than violent insurrection, which would only work once the government is about to fall anyway. It's like the cherry on top (or the bottom if it's the last to be eaten) rather than the ice cream.
So I think access to guns is largely unnecessary in a country like ours which has a bill of rights/constitution, is internationally known as a defender of human rights and so on. Not that we defend them for other people anymore but generally speaking France, the UN, Australia, Nordic countries etc. would go into a rage fit if the US started killing it's own people and maybe help rebels. We'd be a dramatically different country by then, our bill of rights would be gone etc. but tea partiers think of everything as if next year Obama is going to order the military to shoot anyone who disagrees with him but that's not how it would work at all even if it were possible.