Warhammer 40K Airsoft Conversion

I need help developing a semi-auto/fully automatic airsoft attachment that fits inside a Warhammer 40K Tau Pulse Carbine shell. Key Requirements: - Gearbox should allow switching between semi-auto and fully automatic - Design must be accurate to the Warhammer 40K Tau Pulse Carbine - Outer shell should be made of high-density plastic Ideal Skills and Experience: - Experience with airsoft gun modification - Strong design skills for accuracy in replicas - Proficiency in working with high-density plastic materials