An AR 15 modified with a bump stock is capable of firing at 500 rounds per minute
According to who? Bushmaster, a (the? not sure, doesn't matter) manufacturer of the AR-15, seems to disagree with your statement. Do you propose to know more about the AR than them?
Why anyone needs that firepower beyond me
Unless you are planning on a massacre
Or you are wanting the ability to defend yourself from, oh, I don't know, a tyrannical government, for example. I am not suggesting that is a real threat today in the U.S., however, can you name just one society that has descended into tyranny with a well armed populace, capable of defending itself?
There does seem to be a fundamental difference in the way government is viewed here as well. Some seem to believe the government is there to keep us "in line". I see it the other way around. WE are there to keep the government "in line". The moment the government no longer fears the populace, is the moment we start to lose our rights, liberties, and freedom.