well, for one, the constitution gives us that RIGHT. 2nd, what if the government were to turn on the people, and the people have no means of fighting? it is for protection against enemies abroad (foreign gov.) and domestic (our gov.)
you really think having a gun or even ar 15 or fully automatic weapon will protect you from the government if they decide they want your guns or take over some other stuff? You will be fighting against tanks and fighters jets bro, you will die.
Also if a foreign country does invade, they will leave civilians alone, and if you attack them as a civilian they will make sure to murder your ass with a 50 cal machine gun, get over this I need a gun to protect myself from gov't crap, if gov't wanted they will take your guns, you can't stop them, or you'll end up dead.