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