DENVER—Colorado sheriffs upset with new state gun restrictions adopted in the aftermath of last year's mass shootings are trying to invalidate the regulations with a federal lawsuit filed Friday that argues the measures violate the Second Amendment.

At issue are two laws passed this spring that are set to take effect July 1, limiting the size of the of ammunition magazines to 15 rounds and expanding background checks to private and online firearm sales.

The lawsuit involves sheriffs from 54 of the state's 64 counties, and most represent rural, gun-friendly areas of the state where they were elected. They said it was their duty to uphold the U.S. Constitution.

"We each took an oath. The line in the sand has been drawn, and we will stand united," said El Paso County Sheriff Terry Maketa.

In contrast to the sheriffs filing suit, the Colorado Association of Chiefs of Police supported the bills, calling them "common-sense approaches" to protect the public "while not taking guns from law-abiding citizens in any way."

The sheriffs argue the laws have many flaws that will make it nearly impossible for citizens to comply, and they insist the measures won't improve public safety but will restrict the rights of law-abiding citizens.

Colorado lawmakers passed the new gun restrictions in reaction to mass shootings at a suburban Denver movie theater last summer, where 12 people were killed and dozens more injured, and the massacre at an elementary school in Newtown, Conn. The debate over gun control was one of the most emotionally charged issues of the legislative session, with dozens of hours of debate and the White House playing close attention.

Relatives of victims of the Colorado shooting criticized the sheriffs for filing the lawsuit and accused them of playing politics.

"As a parent who lost my son Alex at the Aurora theater shooting, I ask these people to put themselves in my place," Tom Sullivan said in a statement. "I do not understand why these politicians are picking guns over people."

Weld County Sheriff John Cooke said he and his colleagues were "not the ones playing politics with this."

"We believe that the Legislature were the ones who were playing politics," he said.

Opponents of the new gun laws insist the language in the regulations do not provide enough clarity or safeguards to protect people from inadvertently breaking the laws. They've argued that magazines by their nature are easily converted to larger sizes, something the bill bans. They also say the expanded background checks don't provide enough exemptions for temporary transfers, and people conducting private transactions will have a difficult time getting checks through a federally licensed dealer.

Lawmakers allowed several exemptions in the background check, including exempting transfers between immediate family members, shooting events and temporary transfers of 72 hours.

Republican Attorney General John Suthers, who's responsible for defending the state in the lawsuit, issued a statement Friday giving guidance to law enforcement on how the new magazine limit should be enforced. He said magazine features "must be judged objectively" and that magazines that hold 15 rounds or fewer can't be defined as "large capacity" simply because it can be modified to include more.

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