This is a magnetic steel armature designed for the EMG-02 accelerator.  It is compatible with the MG15A magazine.

This ammunition will not fit into the EMG-01B or SGP series accelerators and is not recommended for the GR-1.

Material: Alloy steel

Dimensions: 0.312″ Diameter x 0.75″ Length (each)

Weight: 7.15 grams (each)