Just days after Governor John Hickenlooper signed Colorado's toughest gun-control laws in more than a decade, some sheriffs are denouncing them as unenforceable, even in a state that has seen two of the worst mass shootings in U.S. history.The county-level officers say measures limiting ammunition magazines to 15 rounds and requiring background checks for firearm purchases are impossible to carry out because they're poorly written and don't clearly define the elements of a crime."I've taken a stance that I will refuse to enforce any law that violates peoples' constitutional rights," said Teller County Sheriff Mike Ensminger, west of Colorado Springs."We can't figure out how these laws would be enforced at this point in time without violating someone's constitutional rights," he said.That stance reflects the view of hundreds of rural law-enforcement officials in states from California to Texas to Maryland who publicly proclaimed in the weeks following the massacre at Newtown, Connecticut's Sandy Hook Elementary School that they would defy new gun laws in the name of protecting their constituents' right to bear arms. That divide persists throughout the U.S. as the debate over tightened gun restrictions plays out in Congress and statehouses."The scale of what we're seeing now is unprecedented," said Mark Potok, a senior fellow at the Montgomery, Ala.-based Southern Poverty Law Center. "We have hundreds and hundreds of sheriffs around the country saying they will not enforce any gun control. We have 20 states considering laws that would nullify any gun legislation. That is unconstitutional."The Colorado Association of Chiefs of Police took a different tack, endorsing the new firearms laws, introduced on Feb. 5 and approved six weeks later by the Democrat-controlled legislature over protests from Republicans that they did nothing to improve safety.At a news briefing following a private bill-signing ceremony March 20, Hickenlooper called on county sheriffs to carry out the new measures."I fully expect all of our sheriffs and chiefs of police to enforce the law to the best of their ability," said the governor, 61, a first-term Democrat.The laws were passed eight months after James Holmes was arrested in the deaths of 12 people and the wounding of 58 during a shooting rampage at an Aurora movie theater.