What is the HS code for Doublebell valueline M4?.It is needed to get the permission to buy me a one from abroad.
You’re going to need to be more specific. 
Which country are you in and where do you want to buy it from?
Not readily available unless you are a retailer that uses the 6 digit code to order it would know and none of us are retailers. Besides a few @AKgelblaster may be able to provide that information.
HS code for a water gun is 950300 according to Australia Post Customs declaration form