James B. Vogts, a lawyer for Remington Arms Company, the maker of the gun used in Newtown, argued that that case was not “the place to debate gun laws.”

Mr. Vogts pointed out that the weapons used by Adam Lanza, 20, in the attack on the school were both legally bought by his mother, Nancy Lanza.

He and the lawyers for the other defendants said that the plaintiffs had not met the burden of showing their clients had been negligent and that as a result the case should not be allowed to proceed. They said the question of whether civilians should be allowed to buy and own assault weapons ought to be decided by lawmakers, not the courts.

Judge Barbara N. Bellis of Superior Court asked the lawyers to clarify their argument, wondering why the same rules applied to cigarette manufacturers should not apply to gun makers.

“Cigarettes are legal, but society has changed regarding how safe they may or may not be,” Judge Bellis said. “Why does that same concept not apply to AR-15?”

Mr. Vogts said that while societal norms might change, at the time the weapon was sold to Ms. Lanza, “there was a considered judgment” that such weapons could be lawfully owned in Connecticut. If the case is allowed to proceed, he said, “we’re going to have piecemeal regulation in this country by juries.”

“There is a serious separation of powers issue at play here,” he added, “and it needs to be recognized.”