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This is a 3D printed hatch kit that can be used to modify** your box mag (typically a M249 box mag, but may work with other box mags) to make it easier to fill with BBs. To install, simply hold one of the hatch frame pieces against the box mag to use as a template. Trace the interior of the piece, and mark the 4 holes. Cut the large opening using a Dremel or other suitable tool, and drill 1/8" holes at the corner marks. Sandwich the hatch door between the frame pieces (thick frame on the bottom, thin frame on the top) and screw into place using 4 screws (not provided).