The GHK M4 original part #M4-15 is the original part designed to work with the M4 GBBR. This is a non assembled part so it can be used to completely replace the stock parts or you can use it to replace standalone parts.
The GHK M4 original part #M4-15 is the original part designed to work with the M4 GBBR. This is a non assembled part so it can be used to completely replace the stock parts or you can use it to replace standalone parts.