The judge, William M. Skretny of Federal District Court in Buffalo, called the seven-round limit "an arbitrary restriction" that violated the Second Amendment.

But Judge Skretny said the greater restrictions on assault weapons and high-capacity magazines were constitutional because they served to "further the state's important interest in public safety."

Gov. Andrew M. Cuomo and lawmakers passed the new laws, among the most restrictive in the country, in January in response to the mass shooting at an elementary school in Newtown, Conn., in December.

Gun rights groups have been sharply critical of the measures, holding protests at the Capitol, questioning their legality, and vowing to oppose officeholders like Mr. Cuomo who championed them. The judge's ruling came in response to a lawsuit filed by the New York State Rifle and Pistol Association and other firearms groups.

The seven-round limit on magazines, which Mr. Cuomo, a Democrat, touted when he pushed for the new laws in January, had already run into problems before Tuesday's ruling.

In March, in response to complaints that seven-round magazines were not widely available for sale, Mr. Cuomo said he and legislative leaders would change the restriction so that 10-round magazines could continue to be purchased. But even with that change, gun owners would still be forbidden from loading more than seven rounds into those 10-round magazines.