A gun nut who stockpiled more than 300 illegal firearms at his Long Island home also had rare devices that turn semi-automatic rifles into fully automatic machine guns, officials said today.

Amateur gunsmith Jay Steiner, 66, was arrested in January after turning his Centereach house — across the street from a high school — into an illicit armory.

He had so many weapons that they were strewn haphazardly about his home — officials said they found five locked and loaded guns in a dresser drawer, a shotgun hanging above a closet, and a .38 caliber revolver in the kitchen.

He also offered repair services despite not having a license.

Suffolk County DA Thomas Spota said his office spent months analyzing the arsenal and discovered three devices known as “auto sears” that transform rifles into battle ready machine guns.

The devices are only considered illegal in New York State if attached to a firearm — Steiner had not done so, Spota said. It’s illegal to possess a sear under federal law, but Steiner has not been federally charged.

“Plain and simple — he should not have had these guns and the manner in which he stored these weapons is shocking,” Spota said.

As part of a sting, an undercover officer asked Steiner to fix his revolver and he obliged.