“Unforgivable.” That’s the message a pro-gun rights organization is relaying to former Gov. Eric Greitens as he criticizes Missouri’s newest gun law while running for U.S. Senate. At the heart of the issue is Missouri’s Second Amendment Preservation Act — dubbed SAPA — which took effect in August. It declares federal laws that could restrict gun ownership among law-abiding Missourians as “invalid” and restricts law enforcement officers from enforcing federal firearm regulations that could be deemed invalid under the law. SAPA has garnered national media attention, and some prosecutors and law enforcement officers have expressed concern that the law could...