Because the 2nd Amendment allows for the individual citizen to acquire the same firepower as an individual soldier.
This is the real reason for the 2nd Amendment. There is no apparent "need" today for a military weapon, just as there was no immediate need for a "...well regulated milita..", when the Second Amendment was included in the constitution, they were looking forward in the event there would eventually be the need for men and arms to counter some "need" in the future. The USA will not be a super power forever...no country lasts forever, not even ours. Borders are drawn, re-drawn since recorded history. When ours are in flux, that is when we would need not only semi-autos that look like
assault weapons, but full-autos that are
assault weapons. Or, are you unaware of what has happened in Yugoslavia, Syria, Egypt, Uganda, etc. let alone the Warsaw Ghetto? Is he completely unaware of the "need" for military weapons in his own country after Dunkirk? British naivety almost cost them their freedom then. This is how you answer him.