Hiroshima again. Dropping the atomic bomb on Japan was the right decision. About 75,000 people died almost at once. By December, almost 150,000 would be dead from that single uranium bomb. How many have died since then from lingering effects of the bomb is anyone's guess.
Anyone can build a "gun-type" atom bomb. The only ingredient that is hard to get is fissile material like Uranium 238 or 235. If we don't win the war against the Mohammedan Jihad, you can expect to see one going off in a town near you.