“The Second Amendment is a doomsday provision, one designed for those exceptionally rare circumstances where all other rights have failed—where the government refuses to stand for reelection and silences those who protest; where courts have lost the courage to oppose, or can find no one to enforce their decrees. However improbable these contingencies may seem today, facing them unprepared is a mistake a free people get to make only once.”163 These are sobering remarks”