Exotic Quest: A Voice in the Wilderness (Super Good Advice)

Follow these step-by-step to complete this quest:
 * 1) Find "A Mournful Gun" or "A Recalcitrant Gun" by opening loot chests on Mars
 * 2) Buy an ammunition module from Xur for 1 strange coin
 * 3) Kill 100 enemies with a machine gun to obtain "An Enthusiastic Gun"
 * 4) Bring back to the Gunsmith to claim your reward