So-called "ghost guns" are untraceable firearms that can be bought online and assembled at home, without background checks (and with the aid of YouTube videos). A homemade firearm that federal agents say was recovered on Feb. 6, 2020, from a home in Edmonds, Washington. © U.S. Attorney's Office via AP A homemade firearm that federal agents say was recovered on Feb. 6, 2020, from a home in Edmonds, Washington. Earlier this week, police said one such ghost gun was used in a shooting at a high school in Arizona. The guns' rise in popularity is a "fast-growing scourge" facing the...