In their fight against ISIS, the Kurdish army known as Peshmerga has a secret weapon – a second-generation gunsmith so skilled he can practically turn a bucket of rusty bolts into a killing machine for the rugged, but cash-strapped, fighting force. Bakhtiar Aziz works in a dimly lit basement shop in Erbil, refurbishing guns taken from the enemy, bringing broken and even decades-old firearms back to life and helping to outfit an army as short on weapons as it is long on heart. On a recent day, he inspected an M-16A4 assault rifle badly damaged in a coalition airstrike. Pocked...