Pilot Acroball Ballpoint Pen Black Ink Refills, 0.5mm, BRFV-10EF-B, Set of 6 (Japan import) Refill for Pilot Acroball Ballpoint Pen Black 0.5mm (BAB-15EF-BB) . Smooth oil-based ink "Acro Ink" Ink color: Black The diameter of a ball: 0.5 mm. Fine type.