Authorities found a massive ammo stash in the Hamptons childhood home of Thomas Gilbert Jr., who was charged with fatally shooting his millionaire hedge-fund dad in the head, prosecutors said Thursday.

“Police recently executed a search warrant in his bedroom in his parent’s home in the Hamptons,” said Manhattan Assistant District Attorney Craig Ortner. “Investigators found 114 rounds of live .40-caliber ammunition, and it matched the murder weapon.”

Police also recovered a gun holster and other evidence during the March 16 search, prosecutors said.

Defense lawyers for the trust-fund layabout, who allegedly blasted his father Thomas Gilbert Sr. for cutting his stipend by $200, said they might put on an insanity defense. “I seek to have my client evaluated,” said attorney Alex Spiro.