Athene Arms checks out this BPW MCX SURG URG Upper receiver Kit made for the VFC M4 GBB Series. This product is designed specifically for the VFC M4 GBB series and lower receiver group, intended for Airsoft use only. Due to potential batch tolerances, different lower assemblies may require modifications, so having DIY skills is essential. The Airsoft tracer, which can be installed in a 14mm CCW thread adapter at the front of the tube base, must be less than 38mm in diameter and 125mm in length.