County sheriffs have asked to join the federal lawsuit challenging New York's tough new gun restrictions, calling some provisions vague and impossible to enforce fairly.

The New York State Sheriffs' Association and five individual sheriffs are asking U.S. District Judge William Skretny to add their position to the record. They support gun rights advocates seeking to block enforcement of new bullet limits for magazines and the tighter definition of assault weapons.

The sheriffs agree with the New York affiliate of the National Rifle Association that the law, passed after the Newtown, Conn., school shooting, is unconstitutional because it will prevent citizens from keeping commonly used firearms for home defense.

"The Supreme Court has confirmed that the Second Amendment protects arms typically possessed by law-abiding citizens, and identified that the right of self-defense is 'core' protected conduct that is at its zenith in the home," the sheriffs' brief said. "At a minimum, laws that criminalize the most common rifle in America today - a rifle that is often selected precisely for its self-defense capabilities - impinge upon that core right. The same is true of laws banning standard-capacity magazines."

The law bans magazines with a capacity of more than 10 bullets and generally prohibits loading them with more than seven. It defines assault weapons as semi-automatics with detachable magazines and a single military-style feature such as a pistol grip. The old definition required two such features.

The law also requires New York owners of an estimated 1 million guns, including popular versions of AR-15 rifles now reclassified as assault weapons, to register them by April 15. Since Jan. 15, it has been illegal to sell or buy those guns in the state.

The sheriffs argued that several provisions are also "fatally vague," measures that "law enforcement officers are inherently unable to fairly and uniformly enforce." They urged the court to clarify "laws that will inevitably require enforcement, via confiscation, incarceration, or both, against otherwise law-abiding individuals attempting to exercise fundamental rights."

Without a reliable means of distinguishing each listed assault weapon or now-illegal magazines that "can be readily restored or converted" to carry additional rounds, they said enforcement will be a problem. "The lack of guidelines in these provisions will inevitably lead to 'erratic arrests and convictions' that the due process clause was meant to prevent," they wrote.

Gov. Andrew Cuomo pushed through the law, calling its provisions commonsense responses to limit gun violence. That happened one month after the Newtown shooting, which left 20 schoolchildren and six educators dead. Police said the 20-year-old gunman used a semi-automatic rifle and 30-round magazines.

Defending the state, Assistant Attorney General Benjamin Ahlstrom wrote in court papers last month that the constitutional claims have no merit. He noted the request for an injunction was filed three months after the law passed, undercutting any argument of "perceived imminent harm."