It was definately a good idea at the time , and for several generations an American Citizen was able to purchace wepons as good or better than the US Army could buy. What changed to make this something other than a good idea?
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Obviously, weapons have become far more efficient. Take a machine gun for example.
The government won't let you own a machine gun, grenade launcher, flamethrower, or a surface-to-air missile, even though that is exactly what would be perfect to use to take out a tyrant, flying about in his helicopter. If the 2nd amendment was designed to prevent tyranny, it should allow you to own this sort of weapon.