Colorado’s largest state-based gun lobbying group has filed a lawsuit against Gov. John Hickenlooper over the strict new gun control measures he signed into law in the aftermath of last year’s mass shootings.

Rocky Mountain Gun Owners filed the lawsuit in district court Sept. 4 in hopes of overturning what it says is unconstitutional legislation requiring extended background checks and restricting the size of ammunition magazines.

Dudley Brown, the group’s executive director, said Tuesday the Windsor-based organization “will fight to keep the constitutional rights of Coloradans intact by challenging these laws on every front.”

In May, sheriffs in 55 of Colorado’s 64 counties filed a lawsuit, arguing the law’s limits on ammunition magazines violate the Second Amendment’s right to keep and bear arms. Most of the sheriffs behind the lawsuit represent rural, gun-friendly parts of the state.