This Empty Chamber Indicator (ECI) which sometimes referred to as an Open Bolt Indicator (OBI). The X-Large works with most rifles .30 caliber and above.
WARNING: PROLONGED SHOOTING CAN RESULT IN A HOT BARREL. ALWAYS INSPECT THE EMPTY CHAMBER INDICATOR UPON REMOVAL TO VERIFY THAT NO MELTING HAS OCCURRED AS RESIDUE COULD BE LEFT IN THE CHAMBER OR BARREL.