It happened around noon on Friday on the A/C/E line.
Police say the 18-year-old, from Ohio, was seen charging his phone on the platform.
NYPD officers approached him. They say he had an unloaded AK47 on him and he was also carrying ammunition.
They took him into custody without incident. His identity was not immediately released.
He reportedly told police that he thought it was ok to have the gun and ammo as long as the weapon was not loaded.
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