The long-barreled weapon was found around 3:15 p.m. Friday at the Cortlandt Street Station on the R subway line, about two blocks from the 911 Memorial and Museum, cops said.

“It was an AM15,” an NYPD spokesman said.

The firearm was unloaded with no magazine, police sources said.

The AM-15 is a semi-automatic AR-style rifle produced by Anderson manufacturing. The weapons are frequently used in mass shootings.