A bill signed by Gov. Mike Parson over the weekend — at a gun store called Frontier Justice — threatens a penalty of $50,000 against any local police agency that enforces certain federal gun laws and regulations that constitute “infringements” of Second Amendment gun rights. At least eight other states — Arizona, Arkansas, Idaho, Montana, North Dakota, Tennessee, Texas and West Virginia — have taken similar action this year, passing laws of varying strength that discourage or prohibit the enforcement of federal gun statutes by state and local agents and officers. The new law “is about protecting law-abiding Missourians against government overreach and unconstitutional federal mandates,” Mr. Parson and the attorney general, Eric Schmitt, said in a letter defending the law on Thursday to the U.S. Justice Department. They said the state would “reject any attempt by the federal government to circumvent the fundamental right Missourians have to keep and bear arms to protect themselves and their property.”